120 



EMBRYOLOGY. 



316. As a general fact, it should be further stated, that 

 the envelops which protect the egg, and also the embryo, 

 are the more numerous and complicated as animals be- 

 long to a higher class, and produce a smaller number of 

 eggs. This is particularly evident when contrasting the in- 

 numerable eggs of fishes, discharged almost without protec- 

 tion into the water, with the well-protected eggs of birds, and 

 still more with the growth of young mammals within the 

 body of the mother. 



separate completely the two plates of the amnios (Fig. 126, a), and finally 



Fig. 126. 



to enclose the embryo, with the amnios, in another large sac. The tuhular 

 part of this sac, which is nearest the embryo, is at last transformed into 

 the urinary bladder. The heart (h) is already very large, with minute 

 arterial threads passing off from it. 



315 d. The development of mammals exhibits the following peculiari- 

 ties. The egg is exceedingly minute, almost microscopical, although com- 

 posed of the same essential elements as those of the lower animals 

 The vitelline membrane, called chorion, in this class of animals, is 

 comparatively thicker (Fig. 127, v,) always soft, surrounded by peculiar 



cells, being a kind of albumen. 

 The chorion soon grows proportion- 

 ally larger than the vitelline sphere 

 itself (Fig. 123, y), so as no longer to 

 invest it directly, being separated 

 from it by an empty space (&). The 

 germ is formed in the same position 

 Fig. 127. Fig. 128. as in the other classes of Vertebrates, 



namely, at the top of the vitellus (Fig. 129) ; and here also two layers 

 may be distinguished, the upper or serous layer (s), and the lower or 



