THE AMNION. 247 



to this extent, that the space between the two layers of the 

 amnioii gives the allantois a ready opportunity for free and 

 rapid growth, and enables it to obtain a position close to the 

 inner surface of the egg-shell (Figs. 100, 101, TA), where its 

 respiratory efficiency is greatest. 



It would, however, not be right to regard the amnion as 

 merely a provision to insure free growth of the allantois, for 

 this would not explain how the amnion originated in the first 

 instance ; and it must be remembered that all the characteristic 

 stages in the development of the amnion are completed while 

 the allantois is still in a very rudimentary condition. The 

 anmioii has probably to be explained quite irrespectively of the 

 allantois. 



The most satisfactory explanation of the formation of the 

 amnion is that it is due, in the first instance, not to uprising 

 of a fold of somatopleure, but to depression of the embryo 

 into the yolk-sac ; the sinking of the embryo being due partly 

 to its own weight, partly to the downward growth of the 

 front part of the head caused by cranial flexure ; and perhaps 

 in part to the resistance of the vitelline membrane, aided by the 

 liquefaction of the yolk as this becomes absorbed for the 

 nourishment of the embryo. The main purpose effected by the 

 depression of the embryo is to remove it from the danger of 

 pressure against the egg-shell, a consideration which has more 

 weight in the case of Reptiles, in which group the amnion was 

 first acquired, and in which the yolk often completely fills the 

 egg-shell, than in their descendants, the Birds. 



THE DEVELOPMENT OF THE NERVOUS SYSTEM. 



1. General Account. 



The development of the nervous system of the chick is 

 effected in practically the same manner as that of the frog. 

 About the nineteenth or twentieth hour, almost immediately 

 after the notochord has appeared, the epiblast in front of the 

 primitive streak becomes thickened along the median line to 

 form the neural plate (Fig. 107, xr). 



During the next four or five hours, the anterior part of the 

 area pellucida grows rapidly (Fig. 109) : the neural plate lengthens 

 with it, and soon becomes considerably longer, and more promi- 



