EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 47 



ing food for itself, whereas the mammalian embryo is nourished from the 

 blood of the mother almost from the outset of development. 



Inspection of Fig. 4.1 reveals the great similarity of the early embryonic 

 stages of all the forms shown. The embryos in the second and third hori- 

 zontal rows, from the bottom, are so similar that only an expert could tell 

 them apart if they were misplaced. By the stage represented in the fourth 

 row, the fish and salamander have acquired more identifying character- 

 istics, but even in the stage represented by the fifth row the similarities of 

 embryonic lizard, opossum, monkey, and man are most striking. We see, 

 then, that the embryos of these diverse animals all follow a common 

 pattern at first but progressively diverge from this pattern as they approach 

 their respective adult morphologies. As von Baer, pioneer embryologist of 

 the last century, expressed it, "During its development an animal departs 

 more and more from the form of other animals" (as translated in de Beer, 

 1958). 



Why do we find evidence of common pattern in embryonic development? 

 This is the same question asked in the preceding chapter about common 

 pattern in adult structure, and the answer is similar. The common pattern 

 of embryonic development seems most reasonably explained as having 

 been inherited from an ancestor common to all the animals possessing the 

 similar embryonic developments. Explanations not involving common an- 

 cestry may take two forms. It may be maintained that the Creator created 

 each species separately but saw fit to confer on diff"erent species similar 

 processes of embryonic development. Or it may be maintained that me- 

 chanical and physiological necessities operating in development bring 

 about the similarities — that there is, in elTect, no other road which an 

 ovum could follow in its development to the adult state. An apparent in- 

 stance of this is discussed below (p. 68). Similar physical forces un- 

 doubtedly have similar efi'ects in producing basic similarities among em- 

 bryos. Yet detailed similarities in development, like those to be considered 

 presently, seem not to be completely explained as the result of such 

 similar forces. 



Homology in Early Development 



The earliest stages of embryonic development, even in much more di- 

 verse forms than those included in Fig. 4.1, which after all are all verte- 

 brates, are remarkably similar — so much so that it is possible to design a 

 "typical" diagram of early stages in development. Fig. 4.2 shows typical 

 development of an ovum containing little yolk, as for example starfish 



