152 



EMBRYOLOGY. 



contrary, in the mammals, the chorion, which cor responds 

 to the vitelline membrane, is vivified, and finally becomes 

 attached to the maternal body, thus establishing a direct con- 

 nection between the young and the mother ; a connection 

 which is again renewed in another mode, after birth, by the 

 ^rocess of nursing. 



similar to those described in birds : its body and its organs are 

 formed in the same way ; an amnios encloses it, and an allantolfs 

 grows out of the lower extremity of the little animal. As soon as the 

 allantoYs has surrounded the embryo, its blood vessels become more 

 and more numerous, so as to extend into the 

 fringes of the chorion, (Fig. 131, pe;) while, 

 on the other hand, similar vessels from the 

 mother extend into the corresponding 

 fringes of the matrix, (p m,) but without 

 directly communicating with those of the 

 ehorisn. These two sorts of fringes soon 

 become interwoven, SD as to form an intri- 

 cate organ filled with blood, called the pla- 

 centa, to which the embryo remains sus- 

 pended until birth. 



315 /. From the fact above stated, it is clear that there are three 

 modifications of embryonic development among vertebrated animals, 

 namely, that of fishes and naked reptiles, that of scaly reptiles and 

 birds, and that of the mammals, which display a gradation of more and 

 more complicated adaptation, in fishes and the naked reptiles, the 

 germ simply encloses the yolk, and the embryo rises and grows from 

 its upper part. In the scaly reptiles and birds there is, besides, an 

 amnios arising from tie peripheric part of the embryo and an allantoYs 

 growing out of lie iower cavity, both enclosing and protecting the 

 germ. 



Fig. 131. 



