[Transcriber's Note:

 Superscript numbers are shown by ^. A few minor printing errors are listed at the end of the text. ]

 Smithsonian Miscellaneous Collections Volume 56 Number 11

 DEVELOPMENT OF THE DIGESTIVE CANAL OF THE AMERICAN ALLIGATOR

 With Fifteen Plates

 By ALBERT M. REESE Professor of Zoology, West Virginia University

 [Illustration: Smithsonian symbol]

 (Publication 1946)

 City of Washington Published by the Smithsonian Institution 1910

 The Lord Baltimore Press BALTIMORE, MD. , U. S. A. 

DEVELOPMENT OF THE DIGESTIVE CANAL OF THE AMERICAN ALLIGATOR

By ALBERT M. REESE

Professor of Zoology, West Virginia University

In a previous paper (6) the writer described the general features in thedevelopment of the American Alligator; and in other papers specialfeatures were taken up in more detail. 

In the present paper the development of the enteron is described indetail, but the derivatives of the digestive tract (liver, pancreas, lungs, etc. ) are mentioned only incidentally; the development of theselatter structures may be described in a later paper. 

No detailed description of the histological changes taking place duringdevelopment has been attempted, though a brief description of thehistology is given for each stage discussed. 

The material upon which this work was done is the same as that used forthe preceding researches. It was collected by the author in centralFlorida and southern Georgia by means of a grant from the SmithsonianInstitution, for which assistance acknowledgment is herewith gratefullymade. 

Various methods of fixation were employed in preserving the material. Inpractically all cases the embryos were stained in toto with BoraxCarmine and on the slide with Lyon's Blue. Transverse, sagittal, andhorizontal sections were cut, their thickness varying from five tothirty microns, depending upon the size of the embryos. 

The first indication of the formation of the enteron is seen in the veryearly embryo shown, from the dorsal aspect, in figure 1. The medullaryfolds and notochord are evident at this stage, but no mesoblasticsomites are to be seen. 

A sagittal section of approximately this stage, shown in figure 1A, represents the foregut, _fg_, as a shallow enclosure of the anteriorregion of the entoderm, while the wide blastopore, _blp_, connects theregion of the hindgut with the exterior. No sign of a tail fold beingpresent, there is, of course, no real hindgut. The entoderm, which hasthe appearance of being thickened because of the fact that the notochordhas not yet completely separated from it, is continuous, through theblastopore, with the ectoderm. Posterior to the blastopore the primitivestreak, _ps_, is seen as a collection of scattered cells between theectoderm and the entoderm, apparently formed by proliferation from theventral side of the ectoderm. 

A slightly later stage is shown in figure 2, a dorsal view of an embryowith five pairs of mesoblastic somites. A sagittal section of this stageis shown in figure 2A. The foregut is here more inclosed, and thenotochord, _nt_, having separated from the entoderm, _en_, is seen as adistinct layer of cells extending from the foregut to the blastopore. 

A transverse section through the headfold of this stage is shown infigure 2B. The foregut is seen as a wide cavity, _ent_, depresseddorsally, apparently, by the formation of the medullary groove and thenotochord; it is wider laterally than in a dorso-ventral direction, andits walls are made up of about three layers of closely arranged, irregular cells; the wall is somewhat thinner on the dorsal side, justbelow the notochord. 

Figure 3 is a dorsal view of the next stage to be described; aboutfifteen pairs of somites are present. 

Figure 3A is a transverse section through this embryo near the anteriorend of the enteron, _ent_, which cavity, cephalad to this region, isbluntly pointed. As seen in the figure the enteron is here wide fromside to side, and is depressed dorso-ventrally except for a wide groovein the ventral wall. This groove is lined with rather more closelyarranged cells, and marks the region where the mouth will break throughat a somewhat later stage. A short distance caudad to this region thegroove disappears and the pharynx is reduced to a shallow slit extendingalmost to the superficial ectoderm on either side; then the slit-likepharynx becomes suddenly reduced in a lateral and increased in adorso-ventral direction, to assume the outline shown in figures 3B and3C. At a point about one-third of the length of the embryo from the tipof the head, the enteron opens to the yolk-sac, so that what now may becalled the foregut has this considerable extent. There is, however, notthe slightest indication of a tail-fold, so that there is no inclosedhindgut at all. As is shown in figure 3D, the neurenteric canal, _nc_, still opens ventrally, though the medullary canal, _mc_, has now nodorsal opening to the exterior. The medullary canal continues for ashort distance (about fifteen sections of five microns thickness)posterior to the opening of the neurenteric canal. 

Figure 4 is a surface view of the next stage to be described. There arehere about twenty pairs of somites, though the exact number cannot bedetermined. Although not visible externally in the surface view shown, the gill clefts are beginning to form, and the first one opens to theexterior as will be seen in sections of another embryo of this stage. The mouth has now broken through, putting the wide pharynx intocommunication with the exterior; probably the mouth opening is formed atabout the time of the opening of the first gill cleft. 

Figure 4A represents a transverse section through the head of an embryoof the approximate age of the one just described; it passes through bothforebrain, _fb_, and hindbrain, _hb_; through the extreme edge of theoptic vesicles, _ov_, and through the anterior end of the notochord, _nt_. It is just cephalad to the anterior end of the pharynx and to thehypophysis. The chief purpose in showing this section is to representthe two large head-cavities, _hc_. The origin of these cavities may bediscussed at a later time. They are irregularly oval in cross section, and extend in an antero-posterior direction for a distance about equalto their long axis as seen in cross section. The two cavities projecttowards each other in the middle line, and are almost in contact withthe notochord, in the region figured, but they do not fuse at any point. These two head-cavities are the only ones to be seen, in this animal, unless the small evaginations from their walls represent other cavitiesfused with these. Their walls are thin but distinct, and consist of asingle layer of cells. These cells are completely filled with theirlarge, round nuclei, so that the wall has the appearance, under highermagnification than is used in this figure, of a band of closely strung, round beads. 

Figure 4B represents the eighteenth section caudad to the one justdescribed. It passes through that region of the enteron, _ph_, which maybe called the pre-oral gut, since it lies cephalad to the now openmouth. Owing to the plane of the section the upper angle of the firstgill cleft, _g^1_, is seen on the left, although this would notnaturally have been expected in a section through the pre-oral gut. Theevagination to form the hypophysis, _p_, is seen against the floor ofthe forebrain, _fb_. The wall of this region of the enteron iscomparatively thin, and consists of not more than two layers ofcompactly arranged cells with round nuclei. 

Figure 4C is about forty sections caudad to the one just described. Itpasses through the mouth, seen as a vertical opening between the twomandibular arches, _md_. The hyomandibular cleft, _g^1_, the only onewhich opens to the exterior in this embryo, is very wide, and may betraced through a number of sections; in this section the opening is seenonly on the left. The pharynx, _ph_, is very wide; as it is followedcaudad its ventral opening is gradually closed by the approach of thetwo mandibular folds. The dorsal wall of this region of the pharynx isvery thin, consisting of a single layer of flat cells with round nuclei;while the ventral wall, leading through the mouth and lining themandibular folds, is composed of two or three layers of compactlyarranged cells. 

Figure 4D is through a plane sixteen sections caudad to the last. Inthis region, which is just caudad to the otic vesicles, the pharynx hasstill its rectangular outline, and its walls are of the same characteras in the preceding figure. The posterior edges of the hyomandibularclefts are seen projecting in a ventro-lateral direction, _g^1_; whiledorsal to these are the wider, second pair of clefts, _g^2_. Where themandibular folds come together posterior to the mouth, they fuse firstat their outer or ventral border, which leaves a deep, narrow groove inthe anterior floor of the mouth. As this groove is followed caudad itsventral wall is seen to become much thickened, _tg_, to form the_anlage_ of the thyroid gland. In the present section the walls of thegroove are just fusing, to cut off the cavity of the gland from thedorsal part of the groove. The next section caudad to this shows thethyroid as a round, compact mass of cells, with a very small lumen, still closely fused with the bottom of the oral groove. The lumen may, in this embryo, be traced for only a few sections, caudad to which thethyroid is seen as a small, solid mass of cells unattached to the oralgroove. Close to the sides of the thyroid are seen two large bloodvessels, _ar_, the mandibular arches, which unite into the singleventral aorta just caudad to the posterior end of the thyroid. Highpower drawings of the thyroid just described are shown in figures 4Eand 4F. 

Figure 4G is about fifty-five sections caudad to the preceding figure, and passes through the middle region of the heart, _ht_. The enteron, _ent_, is cut caudad to the last gill cleft, but it is nearly as largeas in the pharyngeal region described above; its walls are of a moreeven thickness than in the more anterior sections, though there is anarea, just below the aorta, where the wall is still but one cell thick. In the ventral wall of this part of the enteron, and, to some extent, inthe lateral walls, there seems to be a tendency for the nuclei to becomecollected toward the side of the wall away from the digestive cavity;this condition cannot be well seen in the figure owing to the amount ofreduction in reproduction. 

Figure 4H is seventy-nine sections posterior to the last, and passesthrough the foregut, _ent_, just cephalad to the anterior intestinalportal and caudad to the heart. The outline of the enteron is herealmost a vertical slit, and the lining entoderm consists, in its dorsaland lateral regions, of a single layer of columnar epithelium, while inits ventral region, where it adjoins the liver trabeculae, it is made upof several layers of cuboidal or irregular cells. The nuclei in thedorsal and lateral regions of the entoderm are arranged in a verydefinite layer at the basal ends of the cells, though an occasionalnucleus may be seen near the center of the layer. The mesoderm thatextends ventrad from the mesentery, on each side of the entoderm justdescribed, consists of a thick layer of compactly arranged cells. Theventral end of the entodermal wall is fused with the wall of a smallcavity, _li_, which may be traced several sections cephalad to thisplane. This cavity is a part of the system of hollow liver trabeculaeseen as a group of irregular masses of cells ventrad to the enteron atthe opening of the anterior intestinal portal. The large blood vessel, _bv_, is the meatus venosus. 

Figure 4I is just four sections caudad to the preceding. It passesthrough the anterior intestinal portal, _aip_. The medial livertrabecula into which the enteron was seen to open, in the precedingfigure, now opens ventrally to the yolk-sac as the anterior intestinalportal. A few liver trabeculae are to be seen on either side of theportal, but they show no lumena, and may be traced through only a fewsections. The extent of this uninclosed region, the midgut, is verydifficult to determine with accuracy, but, at this stage, it comprisesabout one-half of all the sections of the series. The difficulty is duepartly to the unavoidable tearing of the tissues in removing the embryofrom the yolk-sac, and partly to the indefiniteness of the posteriorintestinal portal, where the walls of the enteron are very thin. As seenin figure 4I the location of the anterior intestinal portal is verydistinct. 

A short distance caudad to the anterior intestinal portal there isconstricted off from the roof of the midgut a narrow diverticulum, figure 4J, _i_, the meaning of which is not apparent; it extends throughonly ten to fifteen sections, tapering caudad till it disappears. Theregion of the hindgut, at this stage, is about one-fifth of the entirelength of the embryo. Its anterior portion is wide and, as has beensaid, rather indefinite in outline. 

Figure 4K represents a typical section through the midgut region of anembryo of about the age of the one from which the preceding figures weredrawn. This and the following figures of this stage were drawn from anembryo in which the posterior region was in better condition than in theembryo from which the other figures of the stage were taken. Themesentery, _ms_, is here of considerable length and continues around theyolk in a layer of diminishing thickness. The epithelium of this regionof the enteron consists of a single layer of fairly regular cells, whichare columnar in the dorsal region, just beneath the mesentery, andcuboidal or even flattened in regions more distant from the medianplane. 

Figure 4L, through the region of the hindgut, shows at _i_ thecompletely inclosed intestine; it is a comparatively narrow tube, linedwith columnar epithelium outside of which is a dense layer of mesoblastcontinuous with the mesentery. In the center of the figure theallantois, _al_, is seen as an irregular cavity, lined with a singlelayer of columnar or cuboidal cells, and surrounded by a thick mass ofloosely arranged, stellate mesoblast cells. The allantois is probablysomewhat larger here than in the other embryos used for this stage, inwhich it was torn away. The tail, _t_, of the embryo is shown at thelower side of the figure, surrounded by the amnion; it is cut in theregion of a curve so that the caudal intestine, _i_, is cutlongitudinally and has the outline of an elongated ellipse. In thisembryo the caudal intestine could be followed to the end of the tail, through several dozen sections; for some distance posterior to theallantois it is extremely narrow, so that its lumen is almostobliterated, and its walls are made up, in any one place, of not morethan a dozen cuboidal cells. Towards the posterior end of this regionthe intestine is considerably enlarged as seen in figure 4L. 

Figure 4M passes through the region where both the allantois and theWolffian ducts open into the hindgut. The union of the allantois and thegut accounts for the elongated outline of the enteron in this section. The openings of the Wolffian ducts, _wdo_, are seen at the lower end ofthe section of the enteron. The cells lining the Wolffian ducts aresmaller than those lining the enteron. In the lower side of the figureare seen the structures of the tail, including the outline of the tinycaudal intestine, _i_, mentioned above. No sign of a cloacalinvagination could be made out with certainty. 

The next stage to be studied is shown in surface view in figure 5. 

Figure 5A represents a section through the head region of this embryo. Owing to the obliquity of the plane of the section the figure is quiteasymmetrical. The pharynx, _ph_, is lined with a comparatively thinepithelium and opens, on the left, at two places, one the mouth and theother the second gill cleft, _g^2_. In the dorsal wall of this cleft, aswell as in the corresponding wall of the opposite cleft, is seen athickening of the epithelium; these thickenings, _ty_, are the rudimentsof the thymus gland, whose development may be described in detail inanother paper. Compared to the size of the gill clefts the cavity of thepharynx is, at this stage, comparatively small. 

Followed caudad the pharynx becomes depressed until, in the region shownin figure 5B, it is a mere narrow slit, _g_, extending transverselyacross the embryo and opening through the gill clefts to the exterior oneach side. 

Figure 5C passes through the posterior region of the pharynx, _ph_, thetip of the forebrain, _fb_, the anterior edge of the heart, _ht_, andthe curve of the tail, _t_. The chief point of interest in this sectionis the thyroid gland, _tg_. It now lies deep in the tissue of the floorof the pharynx, entirely separated from the pharyngeal epithelium. Itconsists of a compact mass of cells, now showing a bilobed structure inits anterior end, and extending through about twenty-five ten-micronsections. It is solid throughout most of its extent, but, in the sectionfigured, which is near the anterior end, the lobe on the right sideshows a small but distinct cavity scarcely visible in the figure. 

Caudad to the region just described the pharynx contracts suddenly toform the oesophagus, a narrow, V-shaped slit, which soon divides into anupper and a lower cylindrical tube, figure 5D, _ent_. 

Followed caudad the lower of these tubes divides into the two bronchialrudiments, figure 5E, _lu_, which, in the embryo here figured, extendthrough nearly one hundred sections. In the region shown in figure 5Ethe three tubes, _oe_ and _lu_, lie at the angles of an imaginaryequilateral triangle, while in the region of the liver, where thebronchial rudiments end, the tubes lie in the same horizontal plane. 

A short distance caudad to the ends of the bronchial rudiments theoesophagus turns suddenly ventrad and becomes much enlarged to form thestomach, figure 5F, _i´_, which may be traced through twenty-five orthirty sections in this series. The epithelium of the stomach is fairlythick, and consists of five or six layers of compact, indistinctlyoutlined cells with spherical nuclei. Ventrad to the stomach is seen, infigure 5F, a section of the duodenum, _i_, which extends, with graduallydiminishing caliber, for twenty-five or thirty sections caudad to theposterior limit of the stomach, where it opens to the yolk-sac and islost. 

The section that cut this embryo in the posterior region of the stomachalso passed through the hindgut in the region of the posteriorappendages, figure 5G. There the intestine, _i_, is a distinct, cylindrical tube which extends, with not much variation in caliber, andwith little variation in position, from this point to the cloaca. Followed cephalad, towards the posterior intestinal portal, it graduallydiminishes in caliber, as did the foregut on approaching the anteriorintestinal portal. The epithelium consists here of three or four layersof compactly arranged cells, and has about the same appearance as in theoesophagus and duodenum. 

Figure 5H represents a section through the cloacal region, _cl_, showingthe openings into the cloaca of the Wolffian ducts, _wdo_. Just anteriorto these openings the cloaca opens ventrally into a small, anteriorly-projecting pouch, the rudiment of the allantois. 

Caudad to the openings of the Wolffian ducts the cloaca extends ventradas a narrow, solid tongue of epithelium towards the exterior, figure 5I, and fuses with the superficial ectoderm at the caudal end of a prominentridge that lies in the mid-ventral line between the posteriorappendages. In this embryo the cloaca has no actual opening to theexterior; the walls of the part that projects towards the exterior arein close contact, except in the region of the openings of the Wolffianducts, as is shown in figure 5H. 

Owing to the coiling of the end of the long tail the plane of thesection, as is seen in figure 5I, passes through the posterior end ofthe embryo no less than four times. In the most posterior of these foursections of the tail, beginning slightly caudad to the section hereshown, is seen a small cavity which may be called the post-anal gut, _pag_. It has thick walls, and extends for about thirty-five sections inthe series under discussion. Its lumen is very large in its caudalregion, figure 5I, _pag_, and tapers gradually cephalad until itdisappears. Posteriorly the post-anal gut ends quite abruptly not veryfar from the extreme tip of the tail. 

Figure 5J is a composite drawing from reconstructions of the enterons oftwo embryos of approximately this stage. One of these reconstructionswas plotted on paper from a series of transverse sections; the other wasmade in wax from a series of sagittal sections. For the sake ofsimplicity the gill clefts are not represented, and the pharynx, mouth, and liver are represented in outline only. For the same reason the lungrudiment of one side only is shown. 

The relative size of the pharynx, _ph_, as seen in the figure, issmaller than it is in reality because of the small dorso-ventraldiameter (the only one here shown) compared to the lateral diameter. Theend of the lung rudiment, _lu_, is slightly enlarged and lies in a planenearer to the observer than that of the oesophagus, _oe_, though this isnot well shown in the figure. 

The oesophagus, _oe_, diminishes slightly in caliber for a shortdistance caudad to the origin of the lungs, then gradually increases incaliber until it suddenly bends to the side (towards the observer) andmerges into the wide stomach, _i´_. The stomach, which is irregularlyconical in shape, lies in a place slightly nearer the observer than theend of the lung rudiment mentioned above. 

Lying to one side of the stomach and duodenum, and extending cephaladbeyond the end of the lung rudiment is the liver, _li_, whose outline isonly roughly shown here by the broken line. The stomach opens ratherabruptly into the duodenum, _d_, which slopes back towards the plane ofthe oesophagus (away from the observer). 

The projection from the side of the duodenum, _pan_, not well figuredhere, indicates the position of the pancreas, better shown in the nextreconstruction. The duodenum extends only a short distance caudad tothis point and then opens, _aip_, to the yolk-sac. 

The yolk-stalk, or unclosed region of the enteron, is still ofconsiderable extent, though its exact boundaries are not easy todetermine. The distance between the anterior and posterior intestinalportals is approximately shown in the figure under discussion. 

The hindgut is cylindrical in cross section and of about the samediameter throughout, except for a slight enlargement in the cloacalregion. 

The post-anal gut is not shown here; it will be described in connectionwith the next reconstruction where it is figured. 

Figure 6 is a surface view in profile of an embryo of the next stage tobe studied. The manus and pes are here well developed, and the generaldevelopment of the embryo is in considerable advancement over the laststage studied. 

Figure 6A represents a reconstruction, from a series of transversesections, of the enteron of an embryo of about the age of the one shownin figure 6. The outlines of the entire embryo, of the eye, _e_, and ofthe anterior, _aa_, and posterior, _pa_, appendages are shown by brokenlines. Its position being coincident with that of the stomach, liver, and pancreas, the anterior appendage can scarcely be seen. The enteron, including one lung only, for the sake of simplicity, is shaded solidblack, while the liver and pancreas, with their ducts, are outlines withunbroken lines. As in the preceding reconstruction no attempt is made toshow the gill clefts, and only the dorso-ventral profile of the enteronis shown. Caudad to the pharynx, the enteron being more or lesscylindrical in section, this profile gives a good idea of its shape, butin the pharyngeal region, where the lateral diameter is so much greaterthan the dorso-ventral, the reconstruction gives but a poor idea of thesize of that part of the enteron. 

The widely-open mouth, _m_, leads, with no line of demarkation, into thepharynx, _ph_, which is of irregular outline and, as has been said, ofmuch greater lateral than dorso-ventral diameter. 

The pharynx becomes gradually constricted to form the oesophagus, _oe_, a very long and slender structure, which, as will be seen in crosssection, is, at this stage, solid for the greater part of its length. Asin the case of the pharynx, the lateral diameter of the oesophagus isgenerally greater than the dorso-ventral diameter. 

From the floor of the caudal part of the pharynx is pushed out thetrachea, _ta_. In the reconstruction, especially in the anterior end, the trachea appears several times the diameter of the oesophagus; thisis due to the great thickness and indefiniteness of its walls ratherthan to a greater diameter of its lumen. 

At about the position of the line _ta_ the trachea divides into the twobronchi (only one shown in the figure), which are somewhat enlarged atthe ends to form the lung rudiments, _lu_. While the trachea and bronchilie ventrad to the oesophagus, the lungs lie laterad and even dorsad tothe oesophagus and cardiac end of the stomach. Caudad to the heart andin the region of the anterior appendages, _aa_, the oesophagus suddenlyenlarges to form the stomach, _i´_, which has now quite the outline ofthe typical human stomach. 

From the stomach the duodenum, _d_, extends, following a sort ofV-shaped course, towards the yolk-stalk, _ys_. In the region of theyolk-stalk it is somewhat enlarged and ends in a blind sac like acaecum. At the side of this sac is seen the opening of the enteron tothe yolk-stalk; the anterior and posterior intestinal portals are notdistinguishable from each other. From this point the hindgut, _hg_, extends cephalad until it lies laterad to the middle region of theduodenum, then bends through 180° and extends, in an almost straightline, to the cloaca, _cl_, lying in the region of the posteriorappendage, _pa_. 

The allantois, _al_, extends cephalad for some distance from the floorof the cloaca. Some distance caudad to the cloaca, near the end of themuch coiled tail, is seen the post-anal gut, _pag_. This structure ashas been noted above, is quite distinct from the other parts of theenteron. It is of elongated, pyriform outline, with the pointed endextending cephalad. 

In the narrow space between the stomach and the duodenum is theelongated pancreas, _pan_, opening by two or more short ducts into theduodenum. 

The liver, _li_, in the figure under discussion, has about twice thearea of the stomach. It extends caudad and dorsal about the samedistance as the latter organ, but it extends ventrad and cephalad farbeyond the boundaries of the stomach. 

Extending along the ventral border of the liver is a long narrow duct, apparently the bile duct, _bd_. It connects, caudally, with the anteriorend of the pancreas, while at its other extremity, near theantero-ventral corner of the liver; it ends blindly. 

The transverse sections now to be described have been selected from theseries from which the reconstruction, just described, was made. 

Figure 6B represents a typical section through the pharynx. Its plane isapproximately shown by the line 400 of figure 6A though the planeapparently does not cut the eye, _e_. The pharynx, _ph_, has here theoutline of an irregular V. Its walls, except at the outer angles of theclefts, _g^1_, are composed of but a single layer of cells. In thedorsal wall these cells are flattened, while in the ventral wall theyare more rounded. This difference in the shape of the cells accounts forthe slightly greater thickness of the floor over that of the roof of thepharynx. The gill clefts no longer communicate with the exterior. 

Figure 6C represents the caudal half of the embryo in the plane 475 offigure 6A. The section of the pharynx, _ph_, is here crescentic inoutline, and the pharyngeal walls, especially the floor, are somewhatthicker than in the more anterior section just described. Lying a shortdistance dorsad to the pharynx are seen two small, thick-walledopenings, _ty_; these are the rudiments of the thymus glands. They arehere quite distinct from the enteron, and may be traced through a largenumber of sections, being in some regions solid and of a smallerdiameter than in the present section. 

Figure 6D is in the region of the line 500 in figure 6A. The thymusrudiments, _ty_, have about the same appearance as in the precedingfigure, except that they are somewhat larger. The pharynx, _ph_, is muchsmaller than in the last section, and though somewhat crescentic inoutline, its convex side is dorsal instead of ventral in position. Thepharyngeal walls are here thicker, and consist of two or three layers ofcells, instead of the single layer of more anterior sections. 

In the median plane the floor of the pharynx is pushed down, as a solidtongue of cells, _gs_, the anterior edge of the glottis. Ventrad andlaterad to the glottis a crescentic condensation of mesoblast representsthe beginning of the laryngeal cartilages, _la_. 

Two or three sections caudad to the one just described, the two layersof which the tongue of cells from the floor of the pharynx is composedseparate slightly at the bottom to form a small cavity, the trachea, _ta_; this condition is shown in figure 6E, which represents part of asection through the plane 532 of figure 6A. 

The oesophagus, _oe_, is here a solid, crescentic mass of cells, thelumen being completely obliterated. The dorsal part of the tongue ofcells, mentioned above, connects the ventral side of the oesophagus withthe trachea, like a sort of mesentery. Above the oesophagus, on eitherside, is the thymus rudiment, _ty_, in this section practically a solidmass of cells instead of a tube. The epithelium of the trachea hereconsists of three or four layers of compactly arranged cells; thisepithelium is surrounded by a dense mass of mesoblast which isresponsible for the greater thickness of the trachea as seen in figure6A. As has been said, the oesophagus here has no lumen, and whenexamined under high magnification its walls are found to be completelyfused, not merely in close contact. The same is true of the tongue ofcells between the oesophagus and trachea. Two or three sections caudadto the one under discussion this tongue of cells loses its connectionwith the trachea, and the latter structure is entirely independent ofthe oesophagus. 

The solid condition of the oesophagus continues through about fiftysections of this series, the horns of the crescent gradually shorteninguntil only the central part remains as the hollow cylinder seen infigure 6F, _oe_, which is a section through plane 650 of figure 6A. Fromabout this point to its opening into the stomach the oesophagus hasessentially the same structure. Its epithelium is of the simple columnartype, the cells being long, with generally basally located nuclei. 

In the section under discussion the trachea, _ta_, is of about the samesize as the oesophagus, but its epithelium is thicker and consists oftwo or three layers of cells. The trachea extends, as a separate anddistinct structure, through about one hundred and fifteen sections, andthen, at a point four or five sections caudad to the present section, itdivides suddenly into the two bronchial tubes. Each bronchus, like thetrachea, is lined with an epithelium of three or four layers of cells;but the epithelium is surrounded by a thin layer of much condensedmesoblast. The bronchi continue caudad, with slightly increasingcaliber, through about fifty sections, when they suddenly enlarge toform the lungs. As seen in figure 6A the lungs are irregularly conicalin outline and lie on either side of the posterior end of theoesophagus. 

Figure 6G is a section through the plane 750 of figure 6A. Theoesophagus, _oe_, is seen as a small, circular opening between two muchlarger openings, the lungs, _lu_. The epithelium of the oesophagus isthe same here as in the more anterior regions described above; that ofthe lung rudiments is very variable in thickness, even in differentparts of the same section, being in some places composed of a singlelayer of cuboidal or even flattened cells, in other places consisting offour or five layers of cells (not well shown in the figure). Surroundingthe epithelium of the lung rudiments is a thin layer of quite densemesoblastic tissue. A fairly well defined mesentery, _ms_, is nowpresent in this region. 

Filling the greater part of the body cavity, below the oesophagus andlung rudiments, is the liver, _li_; and ventrad to the liver the sectionpasses through a loop of the duodenum, _d_. 

The epithelium of the duodenum consists of four or five layers ofcompactly arranged cells, near the center of an oval mass of fairlydense mesoblast. In a lateral projection of this mass of mesoblast liesa small, circular opening, the bile duct, _bd_. Its epithelium consistsof a single layer of columnar cells. In more anterior sections the bileduct is larger in cross section, being about one-half the diameter ofthe oesophagus. As has been said it ends blindly at a point a shortdistance anterior to the antero-ventral edge of the liver. A fewsections caudad to the one under discussion the bile duct connects withthe liver, figure 6A, _bd´_; and some distance caudad to this the ductopens, _bd´´_, into the duodenum so close to the opening, _pan´_, of thepancreas that it is difficult to determine whether the latter organ hasa separate opening into the duodenum or opens into the bile duct. 

At some distance ventrad to the structures just described the intestineis cut, by the plane of the section, in two places, _i_. The more dorsalof these is inclosed and has, under this magnification, the sameappearance as the duodenum, _d_; a higher magnification, however, showsthat its epithelium consists of a single layer of tall, rather clear, columnar cells. The more ventral of the two sections, above mentioned, which is continuous with the dorsal section a very short distance caudadto this point, is in the region that opens to the yolk--in fact a numberof yolk-granules, _y_, may be seen in the opening. The epithelium ofthis part of the intestine consists of a single layer of clear, columnarcells, which, around the borders of the opening, are thrown intonumerous folds and are almost of goblet form. 

Figure 6H represents a section through the plane 820 of figure 6A. Thesection is caudad to one lung and cuts the extreme tip of the other, _lu_. The liver, _li_, and pancreas, _pan_, are seen at the side of thestomach, _i´_, here cut through its greatest transverse diameter. Theepithelium of the stomach varies somewhat in thickness and consists oftwo or three layers of cells, the variation in thickness being due to avariation in the length of the cells rather than to a variation in thenumber of layers. 

Ventrad to the stomach the intestine, _i_, is cut in three places, ofwhich the most dorsal section is the largest. The epithelium of theseintestinal sections, especially the lower two, consists of usually asingle layer of columnar cells which are clearer than those of thestomach. A fairly thin mesentery, _ms_, supports this region of theintestine. 

In the region of the posterior appendages, _pa_, the section passesthrough the hindgut, _hg_, and allantois, _al_. The former is of aboutthe same size as the more anterior sections of the intestine, but itsepithelium is less clear and is composed of two or more layers of cells. The allantois is cut near its opening into the hindgut; its walls arethin, the epithelium consisting of but a single layer of more or lessflattened cells. 

Figure 7 represents a reconstruction of the enteron of an embryo of 42mm. Crown-rump length. Because of the body flexure and large size of theembryo the head was amputated, in the plane _a-b_, and cut sagitally, while the body was cut transversely in the direction shown by thesection planes. In the present figure the outline of the embryo, including the eye, appendages, and umbilical stalk, is shown by finedotted lines; the outlines of the lungs and liver are shown by heavier, broken lines; while the outlines of the enteron proper and the tracheaare shown in solid lines, filled in which fine stippling. For the sakeof simplicity only one lung and one bronchus are shown. 

Since the head has now quite a reptilian form, the oral cavity, _m_, hasmore or less of the adult outline. A transverse groove near the anteriorend of the lower jaw marks off the tongue, _tn_; and the rudiments ofteeth are seen but not shown in the figure because of the lowmagnification used. 

The pharynx, _ph_, is a very extensive cavity that is sharply separatedfrom the mouth by a prominent transverse fold of skin, the velumpalitum, _vp_, just in front of the posterior nares, _pn_, and by a lessmarked fold from the base of the tongue; it is these two valves thatenable the adult alligator to open its mouth under the surface withoutgetting water into the lungs. The mouth and pharynx are lined at thisstage with a thin, stratified epithelium, which consists of a basallayer of rather tall columnar cells and one or two superficial layers offlattened cells. The pharyngeal epithelium is rather thicker than thatof the oral cavity. 

In the embryo from which this reconstruction was made the pharynx was indirect communication with neither the oesophagus nor the trachea, thoughthe separation in each case was by a mere membrane. The trachea, _ta_, opens, except for this membrane, into the pharynx a short distance backof the transverse, dorsal and ventral folds mentioned above, and almostdirectly ventrad to the posterior nares. The anterior end of theoesophagus, _oe_, is in contact with the extreme postero-ventral wall ofthe pharynx. 

The trachea, which is already surrounded by distinct cartilaginousrings, is long, and of about the same diameter throughout. In the regionof the anterior appendage, at the point marked _X_, it divides into thetwo very short bronchi, which almost immediately open into the lungs, _lu_. The lungs, whose structure will be shown in the sections of thisstage, are large, irregular bodies, extending about equal distancescephalad and caudad to their openings into the bronchi. The caudal endsof the lungs overlap the cephalic end of the liver, _li_. 

The oesophagus, _oe_, is large, and is laterally compressed so that itsdorso-ventral diameter, the one shown in the present figure, is two orthree times as great as its lateral diameter. This gives the impression, in the reconstruction, that the oesophagus is nearly as large as thestomach. 

As has been said, the oesophagus does not open directly into thepharynx, but is separated from it by a membrane which consists of theflattened epithelial layers of both cavities separated by a thin layerof mesoblast. This partition between the pharynx and the oesophagus isnot a mere fold of mucous membrane, but is a complete, though thin, wall, easily seen in the series of sagittal sections from which thisregion of the embryo was drawn. The anterior end of the oesophagus issuddenly constricted so that the actual opening closed by this partitionis not large. 

Followed caudad the dorso-ventral diameter of the oesophagus variessomewhat, as does the lateral diameter, but it remains large throughoutand opens into the stomach with no sharp line of demarkation. Thecharacter of the epithelium of the enteron caudad to the pharynx will bediscussed in connection with the sections to be described below. 

The stomach, _i´_, is very different in outline from what was seen inthe last stage described, figure 6A. Instead of having approximately theform of the typical mammalian stomach it is now so elongated that theopening into the duodenum, the pylorus, _py_, seems to be nearer theanterior than the posterior end. While the position of the pylorus isvery distinct it is difficult to distinguish the line of demarkationbetween the stomach and the oesophagus. 

The extreme caudal region of the stomach is enlarged to form a blindsac, representing the gizzard, _gz_. A slight enlargement in the regionof the pylorus may represent the glandular region of the adult stomach. The stomach opens, in a rather curious way, into the side of theduodenum, _d_, the anterior end of the latter structure having theappearance of a sort of caecum, to be seen in the next stage ofdevelopment. 

The duodenum, _d_, makes a U-shaped bend at the side of the stomach, andthen, in the region of the caudal edge of the gizzard, _gz_, dipssuddenly ventrad and caudad towards the umbilical cord, _u_, where itapparently ends blindly, though this appearance is probably due to anartifact in the embryo from which the reconstruction was made. It islikely that, in removing the embryo from the yolk, the connectionbetween the two loops, _i_, of the intestine was broken. 

The ascending intestinal loop is of slightly less caliber than thedescending loop above mentioned; it passes dorsal and cephalad to theposterior border of the gizzard where its lumen is continuous, for ashort distance, with that of the descending loop above described. Thisunusual condition is probably abnormal, but owing to lack of materialonly one series of this stage was studied. 

At the dorso-caudal angle of the gizzard the small intestine, _i_, opensinto the ventral side of a larger tube which may be called the largeintestine, _il_. The blind end of the large intestine, cephalad to theopening of the small intestine, projects forward, dorsal to the gizzard, as a sort of caecum, _ce_, though this structure is generally stated tobe wanting in the crocodilia, and is not seen in the next stage. 

From the caecum the large intestine passes in a ventro-caudal direction, with gradually decreasing caliber, to the cloaca, from whose anteriorwall the intromittent organ, _io_, projects. 

From the ventral wall of the large intestine, at a point about one-thirdthe distance from the cloaca to the caecum, projects ventrad andcephalad the stalk of the allantois, _al_. Owing to its thin walls andsmall lumen the allantois was traced only a short distance into theumbilical stalk. 

The profile of the liver, _li_, has, at this stage, about the same areaand even outline as that of the lung. It lies, of course, on both sidesof the enteron proper, and overlaps, anteriorly, as has been said, theposterior end of the lung. 

Figure 7A represents a section through the plane 305 of figure 7. A considerable advance in the general development of the organs is seenover the last stage studied. The spinal column is well outlined incartilage, and the ribs are cut at various places, _r_. In the body walla considerable differentiation of muscular tissue has taken place, butit is only faintly shown in this series of figures. The scales, especially along the mid-dorsal line, are shown as an area of lessclosely dotted tissue. 

The lungs, _lu_, cut here through their anterior ends, are large, but donot nearly fill the cavities, _bc_, in which they lie; they have thesacculated appearance characteristic of embryonic lung tissue. 

The oesophagus, _oe_, is cut through about its middle region, where itscaliber is greatest. As was said above, its dorso-ventral diameter ismore than twice its lateral diameter, caused partly by the oblique angleat which it was cut. Its wall, figure 7H, is very thin and exhibits adense layer of mesoblastic tissue, in which circular and longitudinalmuscle layers are beginning to differentiate. It is lined by anepithelium which here consists of a single layer of columnar or cuboidalcells with large nuclei. On the ventral side, where the oesophageal wallis in contact with that of the trachea the epithelium is somewhatthickened by an increase in the number of cell layers. With the lowmagnification used these details could not, of course, be shown. 

The trachea, _ta_, is of much smaller caliber than the oesophagus, especially in its dorso-ventral diameter. While its epithelial lining isnot yet appreciably different from that of the oesophagus, itsconnective tissue wall is much thicker and shows numerous condensations, the rudiments of the cartilaginous rings. In the region represented bythis figure the connective tissue layers of the trachea and oesophagusare continuous with each other, but cephalad and caudad to this pointthey are distinct, though sometimes in contact. Several large bloodvessels, _bv_, on each side of the oesophagus probably represent thecarotids and jugulars, but they were not worked out to determine withcertainty which they were. 

Eighty-five sections (figure 7, _X_) caudad to the one under discussionthe trachea divides into the two bronchi. These bronchi graduallyseparate from each other until, at the point at which they open into thelungs, about eighty sections caudad to their point of separation, theylie on either side of the ventral third of the oesophagus. 

Figure 7B represents a section through the plane 480 of figure 7. Thesection is just cephalad to the heart, and passes through the caudalthird of the lungs, _lu_, which have the same appearance as in thepreceding figure; also through the extreme cephalic end of the liver, _li_. The lungs here much more nearly fill the body cavity than in thepreceding figure. The section being caudad to their openings into thelungs the bronchi do not, of course, show. 

The oesophagus, _oe_, is here of much less diameter than in thepreceding figure, but is still laterally compressed. Its wall issomewhat thicker than in the more cephalic region, the increase beingmainly due to the greater thickness of the connective tissue layer, though the epithelium is also slightly thicker because of an increase inthe length of the lining cells. Instead of lying almost entirely ventradto the lungs, as in the preceding figure, the oesophagus here liesdirectly between them. 

Figure 7C represents a section through the plane 627 of figure 7. Theplane of the section passes through the opening of the stomach, _i´_, into the duodenum, _d_. The cross section of the stomach is somewhatlarger than that of the oesophagus, but it differs from the moreanterior region mainly in the character of its walls. These are muchthicker than in the oesophagus; in the mesoblast which forms the greaterpart of their thickness, muscle fibers are beginning to differentiate. The epithelial layer also is thicker than in the oesophagus; it consistsof tall columnar cells that, at places, are thrown into small folds, figure 7I. These folds, even under the low magnification used, are moreevident than is shown in the present figure. The pylorus, _py_, is wideand, as has been noted in connection with figure 7, is situated farcephalad to the caudal end of the stomach. It opens into the side ratherthan into the end of the duodenum, which projects cephalad as a shortblind pouch, _d_. The stomach and duodenum, in this section, are almostcompletely surrounded by the liver, _li_. 

Figure 7D represents a section through the plane 680 of figure 7. 

The stomach, _i´_, which is cut through its middle region, is somewhatlarger than in the preceding figures, though its walls have about thesame character. Its outer walls are continuous, to a considerableextent, with the tissue of the surrounding body wall, especially in theregion just caudad to the plane of the present section. 

The duodenum, being cut through a double loop (see figure 7), is seen intwo places, dorsally where it is cut through the edge of one loop, andventrally where it is cut square across. In both sections the structureis the same, as might be expected, figure 7J. The surrounding mesoblastis differentiated into muscle fibers, figure 7J, _ml_, which form afairly distinct layer; inside of this layer is a tall columnarepithelium, _ep´_, which is thrown into prominent folds. A thin layer ofmesoblast, probably the submucosa, _sl_, lies beneath the epithelium andprojects up into the folds. About ten or twelve folds are seen in anyone section; only the larger ones are well seen in figure 7D. 

Figure 7E shows a section through the plane 770 of figure 7. It is inthe region of the umbilicus, _u_, and the extreme caudal end of thestomach which has been called the gizzard, _gz_. The small size of thegizzard is due to its being cut near its caudal margin. The enteron ishere cut in no less than seven places: the reason for this will beevident on examination of the plane of the section as shown in figure 7. Dorsal to the gizzard the section cuts the so-called caecum, _ce_, a little nearer its anterior end than is shown in figure 7. Theduodenum, _d_, is cut at five points, and has about the same structureas in the preceding figure. The character of the duodenal loops thatcauses the rather curious appearance of the present figure will bereadily understood by reference to figure 7, though the reconstructionis not mathematically accurate. The ventral projection of the lowerloops of the duodenum into the umbilicus is seen both in the presentfigure and in the reconstruction. The loop of the duodenum that, in thesections, is seen to lie directly ventrad to the gizzard, in thereconstruction is shown too much to the side of the latter organ. Thedescending loops of the duodenum are cut in such a way that thesurrounding mesoblast forms a continuous mass of tissue. 

Figure 7F represents a section through the plane 901 of figure 7. Thesection passes through the kidneys, _k_, the edge of one posteriorappendage, _pa_, the large intestine, _il_, and two regions of the smallintestine, _i_. 

The large intestine is here a thick walled, cylindrical structure, _il_, hanging from a thin mesentery, _ms_, in the much reduced body cavity. The layers of its wall are much more fully differentiated than in themore anterior regions of the enteron. The epithelium is here stratifiedinstead of simple columnar, and the folds into which it is thrown arebroader and less numerous than in the duodenum above described. 

Ventrad to the large intestine, and almost in contact with it, is seenthe allantois, _al_, whose general outline was noted in connection withfigure 7. It is an irregular structure, consisting of a very thin outerlayer of mesoderm, lined with a single layer of flattened epithelialcells. 

Lying at a considerable distance ventrad to the main body of thesection, are seen the two sections of the small intestine, _i_, surrounded by irregular strands of tissue from the umbilicus. Thestructure of these two intestinal loops is about the same as in the moreanterior region described above. 

Figure 7G, the last of this series, represents a section through thecloaca, caudad to the urinary openings, in the plane 1060 of figure 7. The epithelium of the cloaca is, of course, simply a continuation ofthat of the surface of the body, somewhat thickened, perhaps, in thedeeper regions. 

The intromittent organ, _io_, which projects cephalad from the wall ofthe cloaca, is here seen as a three-pointed body of considerable size, projecting ventrally from the body. 

Figure 8 shows in outline the enteron, from the ventral aspect, of anembryo of 20 cm. Total length, or at about the time of hatching. Thedrawing was made from a dissection and, for the sake of simplicity, onlythe enteron, respiratory organs, heart, and thymus are shown. The jaw iscut through on the left side and is turned over to the right, thusbringing into view the roof of the mouth, _m_, and the dorsal side ofthe tongue, _tn_. At the same time the pharynx, _ph_, and the wideanterior end of the oesophagus, _oe_, are cut open, exposing theglottis, _gs_, and vocal cords, _vc_. 

The lungs, _lu_, and trachea, _ta_, which are now fully formed, aredissected loose and drawn over to the right side of the animal, togetherwith the heart, _ht_, and the thymus, _ty_; only one side of the thymusis shown, the other half being hidden by the trachea. 

The mouth has reached nearly the outline of the adult. The lips areformed and, in the anterior part of the lower jaw, four tooth rudiments, _to_, are externally visible. The mucous membrane of the roof of themouth, _m_, is covered with rounded papillae, easily seen with a lensbut not shown in the figure. The tongue, _tn_, is fully formed, and isfree anteriorly and laterally to about the extent that is seen in theadult; the papillae with which it is covered are not so prominent asthose seen on the roof of the mouth. At the base of the tongue is theprominent transverse fold, noted in connection with figure 7, that meetsabove the velum palitinum, not shown here but shown in figure 7. Caudadto these folds is seen the glottis, _gs_, a triangular opening with thevocal cords, _vc_, at its base. 

The mucosa of the inside of the pharynx and the anterior end of theoesophagus, exposed by the dissection, is thrown into numerouslongitudinal folds, not shown in the figure; these well-marked foldsextend throughout the length of the oesophagus. 

The oesophagus, _oe_, tapers gradually from the wide pharynx, _ph_, andthen continues as a cylindrical tube of uniform diameter to the rightside of the anterior end of the stomach, where it opens into the latterorgan. Its walls are thick, and its lumen is almost obliterated by thelongitudinal folds of the mucosa, mentioned above. 

The stomach, _i´_, is oval in outline, though somewhat flattenedlaterally; it is depressed, dorso-ventrally, to a little more than halfthe lateral diameter. As has been said, the oesophagus enters its rightanterior border; the pylorus is on the right side, 3 or 4 mm. Caudad tothe oesophageal opening. The wall of the stomach is comparatively thinexcept in the region of the oesophageal and pyloric apertures, and at apoint, opposite these apertures, on the left side. At the latter pointis an oval or disc-shaped area that is several times as thick as thesurrounding wall; it probably represents the gizzard structure of theadult. The thickening mentioned in the region of the two apertures seemsto be mainly due to a wrinkling of the mucosa which, in other parts ofthe stomach, is nearly smooth, so far as can be seen with the naked eye. A sphincter thickening around the oesophageal and, to some extent, around the pyloric aperture, causes each of these structures to projectinto the stomach like an ileo-caecal valve. 

The pylorus, _py_, opens into a small, pointed, thin-walleddiverticulum, _di_, and, at the same time, into the duodenum, _d_. Thediverticulum noted, also, in connection with figure 7, has relativelythick, wrinkled walls; its significance is not known to the writer. Fromthis diverticulum the duodenum, _d_, leads caudad and laterad for ashort distance as a narrow tube, then suddenly expands into the widestpart of the entire intestine. Into this wide part of the duodenum, 3 or4 mm. From the pylorus, opens the bile duct, _bd_. The bile sac, _bs_, is an elongated oval body with thin walls, lying to the right of thepylorus, its connection with the liver was not seen. 

Lying between the anterior end of the duodenum and the posterior end ofthe stomach, and extending caudad for 10 to 15 mm. , in the median planeof the animal is the pancreas, _pan_. It is a long narrow body of awhitish color; its duct or ducts could not be determined by dissection. The duodenum extends caudad, with gradually diminishing caliber, fromthe enlarged region mentioned above. About 10 to 15 mm. Caudad to thestomach it makes a sort of double loop to the right, a wide loop, _lp_, and a close one, _lp´_, nearer the median plane. From the latter loopthe intestine extends straight to the left, for a distance of about 10mm. , where it makes a small loop cephalad, _lp^2_, and then opens to theyolk-sac, _y_. The yolk-sac is shown here simply as an irregular pieceof tissue, the yolk having been removed. 

The anterior intestinal portal, _aip_, and posterior intestinal portal, _pip_, are in close proximity with each other. 

From the posterior intestinal portal the intestine extends straightcephalad to the posterior end of the stomach, dorsal to which it forms adouble loop, a wider one, _lp^3_, and a narrow one, _lp^4_. From thelatter loop, _lp^4_, the intestine extends straight caudad, parallel andnear to the straight region leading from the posterior intestinalportal, until it reaches the region of the loop _lp^2_, dorsal to whichit forms a small loop, _lp^5_. From loop _lp^5_ the intestine, which ishere of very small caliber, extends caudad for about 10 mm. , where itforms another indistinctly double loop, _lp^6_. 

From loop _lp^6_ the large intestine, _il_, extends, with graduallyincreasing caliber, to the cloaca, _cl_, a distance of 10 to 15 mm. 

Except in the enlarged region near the pylorus the lumen of theintestine is almost obliterated by the folding of its thick walls, sothat little or nothing can be told of its lining with the naked eye. 

A distinct mesentery holds the loops of the intestine in position andbinds the entire enteron close to the dorsal body wall. Because of thelack of properly fixed tissue no sections of the enteron of this stagewere made. 

REFERENCES

1. BRONN, H. G. : Klassen des Thier-Reichs. (Vols. On reptiles. ) 1890. 

2. CHAFFANJON, V. : Observations sur Alligator mississippiensis (Tractusintestinalis und Mesenterium). Ann. Soc. Linn. Lyon, vol. 28, p. 83 ff. , 1881. 

3. EISLER, P. : Zur Kentniss der Histologie des Alligatormagens. Archiv f. Mikr. Anat. , vol. 34, pp. 1-10, 1889. 

4. HERTWIG, O. : Comparative Embryology of Vertebrates. Especiallyvol. 2, pp. 1-241, 1906. 

5. REESE, A. M. : The Nasal Passages of the Florida Alligator. Proc. Acad. Nat. Sc. Phila. , 1901. 

6. REESE, A. M. : The Development of the American Alligator. Smith. Misc. Coll. , vol. 51, No. 1791, pp. 1-66, 1908. 

DESCRIPTION OF FIGURES 1-8, PLATES 1-15

 [Transcriber's Note: The word "Plate" refers to the physical pages on which the Figures were printed. The word is not used as an illustration identifier. Based on the author's age at time of publication, "Miss C. M. Reese" is more likely to have been his sister than his daughter. ]

 The surface views were drawn, under the author's direction, by Miss C. M. Reese. The first two of these views were copied, by permission, from S. F. Clarke; the others were drawn from the specimens themselves. 

 All of the figures of any one stage are given the same number, followed by distinguishing letters, so that it is possible to tell at a glance what figures belong together. 

 All of the figures except those from Clarke were drawn under a camera lucida. 

Figure 1. A surface view of an embryo, from the dorsal aspect, at thebeginning of the formation of the enteron. 

Figure 1A. A sagittal section of an embryo of approximately the age ofthe one shown in figure 1. × 43. 

Figure 2. A dorsal view of an embryo with five pairs of mesoblasticsomites. 

Figure 2A. A sagittal section of an embryo of the stage shown infigure 2. × 43. 

Figure 2B. A transverse section through the headfold of an embryo of thestage shown in figure 2. × 43. 

Figure 3. A dorsal view of an embryo with about fifteen pairs ofsomites. × 20. 

Figures 3A-3D. A series of transverse sections through an embryo of thestage of the one shown in figure 3. × 43. 

Figure 4. A surface view of an embryo with about twenty pairs ofsomites. × (about) 15. 

Figures 4A-4D. A series of transverse sections through the anterior endof an embryo of the approximate age of the one shown in figure 4. × 20. 

Figures 4E and 4F. Two transverse sections through the thyroid gland ofthis stage; more highly magnified. × 102. 

Figures 4G-4M. A series of transverse sections caudad to the preceding. Figure 4H, × 43; other figures, × 20. 

Figure 5. A surface view, in profile, of an embryo at the time of theorigin of the limbs. × (about) 5. 

Figures 5A-5I. A series of transverse sections through an embryo of theage shown in figure 5. × 7. 

Figure 5J. A composite drawing of reconstructions of the enterons of twoembryos of the age of the one shown in figure 5. One reconstruction wasin wax, from sagittal sections, the other was a plotted reconstructionfrom transverse sections. × 14. 

Figure 6. A surface view, in profile, of an embryo with well developedmanus and pes. × (about) 5. 

Figure 6A. A reconstruction, plotted from transverse sections, of theenteron of an embryo of about the age of the one shown in figure 6. × 14. 

Figures 6B-6H. Part of a series of transverse sections from which thepreceding reconstruction was made. × 7. 

Figure 7. A reconstruction of the enteron of an embryo of 42 mm. Crown-rump length. 

Figures 7A-7G. A part of the series of transverse sections from whichthe preceding reconstruction was made. × 7. 

Figure 7H. A high power drawing of a portion of the wall of theoesophagus in the region of figure 7A. × 190. 

Figure 7I. A high power drawing of a portion of the wall of the stomachin the region of figure 7C. 

Figure 7J. A high power drawing of a portion of the wall of the duodenumin the region of figure 7D. 

Figure 8. An outline drawing, from the ventral aspect, of the enteron ofan embryo of 20 cm. Length, at about the time of hatching; made from adissection. × 1. 

LETTERING FOR ALL FIGURES

 _a_, head-fold of amnion. _aa_, anterior appendage. _ac_, anterior cardinal vein. _aip_, anterior intestinal portal. _al_, allantois. _an_, anterior nares. _ao_, aorta. _ar_, aortic arch. _au_, auricle. _b_, bulbus arteriosus. _bc_, body cavity. _bd_, bile duct. _bd´_, opening of bile duct to liver. _bd´´_, opening of bile duct to duodenum. _blp_, blastopore. _bp_, basilar plate. _bs_, bile-sac. _bv_, blood vessel. _c_, centrum of vertebra. _ca_, caudal artery. _ce_, caecum. _ch_, cerebral hemisphere. _cl_, cloaca. _cm_, circular muscle layer. _cn_, cranial nerve. _cp_, posterior choroid plexus. _cv_, cardinal vein. _d_, duodenum. _dc_, ductus Cuvieri. _di_, diverticulum of stomach. _e_, eye. _ec_, ectoderm. _ec´_, thickening of ectoderm. _en_, entoderm. _en´_, endocardium. _ent_, enteron. _ep_, epidermal layer of ectoderm. _ep´_, epithelium. _epi_, pineal body. _es_, embryonic shield. _f_, fronto-nasal process. _fb_, forebrain. _fg_, foregut. _g^1-5_, gill clefts. _gf^1-6_, gill folds. _gl_, glomerulus. _h_, head-fold. _gs_, glottis. _gz_, gizzard. _hb_, hindbrain. _hc_, head cavity. _hg_, hindgut. _ht_, heart. _i_, intestine. _i´_, stomach. _il_, large intestine. _in_, infundibulum. _io_, intromittent organ. _ir_, iris. _it_, iter. _k_, kidney. _la_, larynx. _li_, liver. _lm_, longitudinal muscle layer. _ln_, lens. _lp_, _lp´_, etc. , loops of intestine. _lu_, lungs. _lv_, lens vesicle. _m_, mouth. _ma_, manus. _mb_, midbrain. _me_, medullary canal. _me´_, tip end of medullary canal. _md. _, mandibular folds. _mes_, mesoderm. _mes´_, myocardium. _mf_, medullary fold. _mg_, medullary groove. _mk_, Meckel's cartilage. _ml_, muscle layer. _mp_, muscle plate. _ms_, mesentery. _mv_, meatus venosus. _mx_, maxillary fold. _myc_, myocoel. _n_, nasal cavity. _na_, neural arch of vertebra. _nc_, neurenteric canal. _nl_, nervous layer of ectoderm. _nt_, notochord. _o_, ear vesicle. _oc_, optic cup. _oe_, oesophagus. _on_, optic nerve. _os_, optic stalk. _ov_, optic vesicle. _p_, pituitary body. _pa_, posterior appendage. _pag_, post-anal gut. _pan_, pancreas. _pan´_, opening of pancreas. _pc_, posterior cardinal vein. _pe_, pes. _pg_, primitive groove. _ph_, pharynx. _pip_, posterior intestinal portal. _pl_, pelvis. _pn_, posterior nares. _pr_, pericardial cavity. _ps_, primitive streak. _pt_, pecten. _py_, pylorus. _r_, rib. _rt_, retina. _s_, somites. _sc_, spinal cord. _se_, spenethmoid cartilage. _sg_, spinal ganglion. _sl_, submucosa. _sm_, splanchnic mesoblast. _sn_, spinal nerve. _so_, somatic mesoblast. _st_, stomodaeum. _sy_, sympathetic nervous system. _t_, tail. _ta_, trachea. _tg_, thyroid gland. _tn_, tongue. _to_, tooth anlage. _tr_, trunchus arteriosus. _tv_, _tv´_, third ventricle of brain. _ty_, thymus gland. _u_, umbilical stalk. _v´_, _v´´_, _v´´´_, first, second, and third cerebral vesicles. _va_, vascular area. _vc_, vocal cords. _vm_, vitelline membrane. _vn_, ventricle of heart. _vp_, velum palitum. _vv_, vitelline blood vessels. _wd_, Wolffian duct. _wdo_, opening of Wolffian duct. _wr_, Wolffian ridge. _wt_, Wolffian tubule. _x_, point of origin of bronchi. _y_, yolk. _ys_, yolk-stalk. 

 [Transcriber's Note:

 The Figures were printed on a series of 15 pages ("Plates") at the end of the book. Since the caption text has already been given in full, the individual Figure numbers will not be repeated here. ]

 * * * * * * * * * * * * * *

Errata

 As seen in figure 6A the lungs are irregularly conical _printed as 6a_ at the point marked _X_ _text [two occurrences] unchanged: capital letter not explained_ _ep_, epidermal layer of ectoderm. [. For, ] _epi_, pineal body. [_epi_ not italicized]