172 Vertebrate Embryology 



" Shortly before the time of hatching the allantoic 

 vessels become constricted, by the closure of the body 

 walls at the umbilicus. 



"The allantois itself shrivels up, and is cast off as the 

 chick works its way out of the shell." 



In the highest group of Vertebrates, the 

 Mammals, the allantois becomes developed 

 into a structure known as the placenta, by 

 which the embryo is attached to the parental 

 uterus, and through which it receives nourish- 

 ment from its mother. 



The allantois, then, like the amnion, is an 

 extra-embryonic structure, and is cast aside at 

 the time of hatching. 



The brain. Since the limits of this book 

 will permit the discussion of only the most im- 

 portant points in the development of the brain, 

 and since the development of the important 

 features in the chick is essentially the same as 

 in the frog, the reader is referred to the first 

 part of this book (pages 31-40) for a descrip- 

 tion of the development of the brain. There 

 are some points of difference which might be 

 mentioned ; for example : the cerebellum in 

 the frog remains very small and inconspicuous 

 throughout life, while in the chick it eventually 

 becomes very large, though for the first part of 



