DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 121 



317. But neither in fishes, nor in reptiles, nor in birds, 

 does the vitelline membrane, or any other envelop of the egg, 

 take any part in the growth of the embryo ; while on the 

 contrary, in the mammals, the chorion, which corresponds 

 to the vitelline membrane, is vivified, and finally becomes 

 attached to the maternal body, thus establishing a direct 

 connection between the young and the mother ; a connec- 

 tion which is again renewed in another mode, after birth, 

 by the process of nursing her milk. 



mucous layer (m). As it gradually enlarges, the surface of the cho- 

 rion becomes covered 

 with little fringes, which, 

 at a later epoch, will 

 be attached to the mother 

 by means of similar 

 fringes arising from the 

 walls of the matrix, or 

 organ which contains the 

 embryo. Fig. 129. Fig. 130. 



315 e. The embryo itself undergoes, within the chorion, changes similar 

 to those described in the birds ; its body and its organs are formed in the 

 same way; an amnios encloses it, and an allantois grows out of the lower 

 extremity of the little animal. As soon as the allantois 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 (pm), but without directly 

 communicating with those of the chorion. These 

 two sorts of fringes soon become interwoven so 

 as to form an intricate organ filled with blood, 

 called the placenta, to which the embryo remains 

 p. . . suspended until birth. 



315 f. From the fact above stated, it is clear that there are three modifi- 

 cations 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 the peri- 

 pheric part of the embryo, and an allantois growing out of the lower cavity, 

 both enclosing and protecting the germ. 



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