DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 151 



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. 



317. 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 



Fig. 127. 



Fig. 128. 



passing off from it. At this period there exist true gills upon the 

 sides of the neck, and a branchial respiration goes on. 



315 d. The development of 

 mammals exhibits the following 

 peculiarities. The egg is ex- 

 ceedingly minute, almost micro- 

 scopic, although composed of the 

 same essential elements as those 

 of the lower animals. The vitel- 

 line membrane, called chorion, in 



this class of animals, is comparatively thicker, (Fig. 127, v,) always 

 soft, surrounded by peculiar cells, being a kind of albumen. The 

 chorion soon grows proportionally larger than the vitelline sphere 

 itself, (Fig. 128, y,) so as no longer to invest it directly, being sepa- 

 rated from it by an empty space, (&.) The germ is formed in the 

 same position as in the other classes of Vertebrates, namely, at the top 

 of the vitellus, (Fig. 

 129 ;) and here also 

 two layers may be 

 distinguished, the up- 

 per or serous layer, (s,) 

 and the lower or mu- 

 cous layer, (m.) As 

 it gradually enlarges, 

 the surface of the 

 chorion becomes cov- 

 ered 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. 



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



Fig. 129. 



Fig. 130. 



