MOVEMENTS AND FOLDINGS 49 



Weissenberg's map of the presumptive areas, and 

 the similarity with Vogt's map is immediately 

 obvious. It is an impressive example of the way in 

 which different groups, here the fish and the am- 

 phibia, resemble one another in their early develop- 

 ment. It should be mentioned, however, that the 

 lampreys are a very primitive type of fish ; the more 

 highly evolved types have very yolky eggs and a 

 different sort of gastrulation. 



Birds 



The chick embryo is a classical object for embryo- 

 logical investigations. The first studies we know of 

 were made by Aristotle, and for a very long time 

 no one looked more deeply into the matter than he 

 had. The first sign of development which he could 

 see was the appearance of a pulsating heart which 

 is full of red blood and very large and obvious in 

 the early stages. But gastrulation takes place still 

 earlier, when the embryo is so small that the details 

 can hardly be made out without a lens or a micro- 

 scope. The process is, however, particularly in- 

 teresting, because it is in some ways transitional to 

 the conditions found in mammalian and human 

 embryos. Both birds and mammals are evolved from 

 extinct reptiles, and are therefore related to one 

 another, though rather distantly. 



The bird's tgg^ like the reptile's tg'g, contains a 

 very large amount of yolk and undergoes the dis- 

 ooidal type of cleavage described in the last chapter, 

 ibrming a blastula which consists of a little disc or 



