EVIDENCE FROM EMBRYOLOGY 



63 



while in the frog's egg the yolk is disseminated, 

 though not uniformly, throughout the egg and in the 

 mammalian egg, which is microscopic in size, there is 



FIG. 5. Embryos in corresponding stage of development of 

 Shark (A), Fowl (B), and Man (C). g, gill-slits. 



no yolk. It is a very remarkable fact that all of the 

 vertebrated animals, fishes, amphibians, reptiles, 

 birds and mammals, however different their habits 

 and modes of life, have a mode of ontogeny which is 

 of even more characteristically and unmistakably 

 the same plan than is the type of their adult struc- 



