288 



EMBRYOLOGY. 



Fig. 313. 



Fig. 314. 



another large sac. The tubular 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. At this period there 

 exist true gills upon the sides of the neck, and a branchial 

 respiration goes on. 



475. The development of mammals exhibits the following 

 peculiarties : the egg is exceedingly minute, almost microsco- 

 pic, although composed 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. 313, v\ always soft, surrounded by peculiar cells, being 



a kind of albumen. The 

 chorion soon grows propor- 

 tionally larger than the vitel- 

 line sphere itself (fig. 314, 

 y), so as no longer to invest 

 it directly, being separated 

 from it by an empty space 

 (k). The germ is formed in 

 the same position as in the 

 other classes of the vertebrata, namely, at the top of the vitellus 

 (fig. 315) ; and here also two layers may be distinguished, 



the upper, or se- 



Fig. 315. Fig. 316. rou s layer (s), 



and the lower, 

 or mucous layer 

 (m) . As it gradu- 

 ally enlarges, the 

 surface of the cho- 

 rion becomes co- 

 vered with little 

 fringes, which, at 

 a later epoch, become attached to the mother by means of 

 similar fringes, arising from the walls of the matrix, or organ 

 which contains the embryo. 



476. The embryo itself undergoes, within the chorion, 

 changes similar to those described in birds ; its body and its 

 organs are formed in the same way, an amnios incloses it, 

 and an allantois grows out of the lower extremity of the little 





