AND THE LOWEli ANIMALS. 23 



embryo, a sort of pouch. — the allanto'is — which grows 

 rapidly, and elongating itself like a long-necked bal- 

 loon, is applied against the inner surface of the chorion. 

 This allantois carries with it veins and arteries com- 

 municating with those of the embryo. Moreover, at 

 the point of contact the vital activity of the mem- 

 brane is increased. The villi of the chorion grow 

 rapidly and become more numerous. Finally, the egg 

 grafts itself upon the uterus of the mother, being- 

 retained there till the time of its birth ; and from this 

 period it receives nutrition at the expense of the 

 parent. 



While the transformations that I have been 

 alluding to are going on, the germ itself is not 

 inactive. 



We have seen, that at the period to which I have 

 referred, this germ is composed of modified yolk 

 embraced by the blastodermic membrane, upon a 

 limited portion of whose surface the germinal area 

 and rudiments of the embryo are borne. In pro- 

 portion as the latter assumes its special characters, 

 and as the walls of its great cavities close in, it 

 gradually withdraws from the surface of the sphere 

 which contains it, and remains attached only by a 

 sort of canal. At the end of a certain period the 

 embryo and blastodermic vesicle are only united by a 

 kind of hollow cord and a few blood-vessels, some- 

 what (if I may be excused the comparison) like the 

 bowl and handle of a cup and ball. From this time 

 the blastodermic vesicle is styled the umbilical vesicle. 

 The latter in some cases becomes atrophied, and dis- 

 appears, as in man and the ruminants ; in others, on 

 the contrary, as in carnivora and rodentia, it increases 



