DEVELOPMENT OF THE YOUNG WITHIN THE EGG. Ill 



and a new skin is supplied by the transformations of the 

 blood. Similar changes take place in the embryo, during 

 its early life ; only, instead of being limited to a part of 

 the body, they pervade the whole animal. 



295. The series of changes commences, in most animals, 

 soon after the eggs are laid ; in others, the birds for 

 example, they are delayed till the commencement of 

 incubation. The yolk, which before was a mass of uni- 

 form appearance, now begins to present a diversified 

 aspect. Some portions become more opaque, and others 

 more transparent ; and the germinal vesicle, which was 

 in the midst of the yolk, is seen at the upper part of it, 

 where the germ is to be formed. These early changes 

 are accompanied, in some animals, by a rotation of the 

 yolk inside of the egg, as may be distinctly seen in the 

 eggs of some of the mollusks, especially of the snails. 



296. At the same time the yolk divides itself into two 

 spheres, which are again regularly subdivided into two more, 

 and so on, till the whole yolk assumes the form of a mulberry, 

 each of the spheres composing the mulberry having in its inte- 

 rior a transparent vesicle. In many animals, however, these 

 divisions or fissures are only temporary, and seem to be mere- 

 ly a peculiar mode of transformation common to all inverte- 

 brate animals, and also to fishes, naked reptiles, and mam- 

 mals, but not yet observed in birds and the higher reptiles.* 



297. In the next place, there appears upon the yolk of 

 the Vertebrates a disc-shaped protuberance, composed of 

 little cells, which has been variously designated under 

 the names of germinative disc, proligerous disc, blasto- 

 derma, germinal membrane, or simply the germ. This 

 disc gradually extends itself, until it embraces the whole, 

 or nearly the whole, of the yolk. 



* In the Birds and higher reptiles we find, instead, a peculiar organ called 

 cicatricula, which may, nevertheless, have been formed by a similar pro- 

 cess before the egg was laid. 



