152 EMBRYOLOGY, 



contrary, in the mammals, the chorion, which corrcspc ids 

 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 

 process of nursing. 



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

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

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

 allantoTs has surrounded the embryo, its blood vessels become more 

 and more numerous, so as to extend into the 



fringes of the chorion, (Fig. 131,;? e ,-) while, 'Pm 



on the other hand, similar vessels from the ^[ji^^^ff^^^^ 

 mother extend into the corresponding 

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

 directly communicating with those of the 

 chorisn. These two sorts of fringes soon 

 become interwoven, sd as to form an intri- 

 cate organ filled with blood, called the pla- Fig. 131. 

 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 adiiptation. In fishes and the naked reptiles, the 

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

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

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

 growing out of he lower cavity, both enclosing and protecting the 

 germ. 



