254 



PRINCIPLES OF ANIMAL BIOLOGY 



be alike in so many respects unless their evolutionary histories were 

 largely the same. 



Homology Ascertained from Embryonic Development. — Somewhat 

 better evidence of homology than is afforded by adult structure can often 

 be obtained from a knowledge of embryonic development. As was 

 pointed out in Chap. 16, corresponding structures in vertebrate animals 

 arise in essentially the same way in the embryo. The nervous system of 

 one vertebrate begins with ridges that are much like those of another 

 vertebrate embryo. The eye of a bird develops as does the eye of a frog. 

 The early ear also is about the same, whether found in a reptile or a 



/7s/7 Salamander Tortoise Chick 



Hocf Calf Rabbit Man 



Fig. 209. — Homology of embryonic form, and particularly of gill clefts and bars, in verte- 

 brates. {From Haupt, after Romanes, "Darwin and After Darwin.") 



mammal. This similarity of the first appearance of embryonic structiu"es 

 occurs even when the adult organs are strikingly diffei'ent. The arm of a 

 man and the wing of a bird are different from one another in the adult 

 condition, especially in the hands; but in the embryo the earliest limb 

 l)uds are almost identical. An even greater difference exists between the 

 adult fore- and hind limbs of a bird. When compared, bone for bone, 

 there is scarcely a point at which there is not a distinct difference. Yet 

 the wing and leg could be interchanged in the early embryo, and few 

 observers would detect the substitution. Even the general form of the 

 whole embryo is similar in the several classes of vert(^brates (Fig. 209). 

 This illustration also shows the common origin of gill clefts and gill bars, 

 and their presence in the embryos of reptiles, birds, and mammals which 



