THE EGtt IS THE OVIDUCT. 



289 



Fig. 317. 



animal. As soon as the allanto'is has surrounded the embryo, 

 its blood-vessels become more and more numerous, so as to 

 extend into the fringes of the chorion 

 (fig. 317, p, e), while, on the other 

 hand, similar vessels from the mother 

 extend into the corresponding fringes 

 of the matrix (p, m), but without di- 

 rectly 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 suspended 

 until birth. 



477. From the fact above stated, it is clear that among 

 the vertebrated animals there are three modifications of em- 

 bryonic development, namely, that of fishes and naked reptiles, 

 that of scaly reptiles and birds, and that of 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 peripheral part of the embryo, and an allanto'is grow- 

 ing out of the lower cavity, both inclosing and protecting the 

 germ. 



478. As a general fact, it should be further stated, that 

 the envelopes protecting the egg, and also the embryo, are 

 the more numerous and complicated as animals belong to a 

 higher class, and produce a smaller number of eggs. This is 

 particularly evident when contrasting the innumerable eggs $f 

 fishes, discharged almost without protection into the water, 

 with the well-protected eggs of birds, and still more with the 

 growth of young mammals within the body of the mother. 



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

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

 take any part in the growth of the embryo ; while, on the con- 

 trary, in mammals, the chorion, which corresponds to the 

 vitelline membrane, is vivified, and finally becomes attached to 

 the maternal body, thus establishing a direct connection be- 

 tween the young and the mother : a connection which is 

 again renewed in another mode, after birth, by the process of 



nursing. 



u 



