EXERCISE IX 

 HOMOLOGY 



Structures or organs having a similar embryonic origin irrespective of 

 their final form or their function are held 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 direc- 

 tions. 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, lame rounded 

 prominences on either 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. 



26. Examine a chick embryo after 100 to 120 hours of incubation. 

 The general features may be recognized from the description above. 

 In addition observe that the limb buds are elongated, that their distal ends 

 are flattened, and that the division into digits has begun. (The latter 

 feature is best observed if the light falls obliquely on the flattened surface 

 of the limbs, so as to throw shadows in the hollows.) 



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