Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: EXERCISE IX HOMOLOGY Structures or organs having a similar embryonic origin irrespective of their final form or their function are said to be homologous. All such structures are believed to have been derived from some common ancestral, more generalized type of structure and to have diverged in various directions. The generalization that the ancestral history of a structure is repeated during embryonic development is known as the Biogenetic Law or the Recapitulation Theory. It is one of the arguments in favor of the theory of organic evolution. A. EMBRYONIC ORIGIN OF VERTEBRATE LIMBS 1. Limb Buds of a Toad or a Frog Tadpole. la. Examine tadpoles showing a very early stage in the development of the hind legs. Note the rounded prominences, the hind limb buds, at the base of the tail. Draw an outline of the whole animal, two inches long, in side view, and make the limb buds dark. 16. In an older embryo observe that each limb bud is now elongated, and that the distal end is broadened and shows signs of division into several digits. How many? Draw the limb bud, considerably enlarged, without the body. Ic. Determine the number of digits in the hind foot of an adult frog. 2. Limb Buds of Chick Embryos. 2a. Examine a chick embryo after 72 to 80 hours of incubation. Note that the body is in the form of a letter J (see wall chart). The shorter and thicker limb of the J is the head, which bears the eyes, large rounded prominences on each side. The longer and more slender limb of the J is the trunk, the bend of the J being the neck. The limb buds are two semicircular prominences on each side of the trunk. Are they alike? Draw an outline of the body two inches long, and represent the limb buds dark. 2b. Examine a chick embryo after 100 to 120 hours of incubation. ...