300 THE HISTORY OF CBEATION. 



great number of small spheres, naked cells, containing 

 kernels (Fig. 6 D). These cells are the materials out of 

 which the body of the young mammal is constructed. 

 Every one of us has once been such a simple mulberry- 

 shaped ball, composed only of small equi-formal cells. 



The further development of the globular lump of cells, 

 which now represents the young body of the mammal, con- 

 sists first in its changing into a globular bladder, as fluid 

 accumulates within it. This bladder is called the germ- 

 bladder (vesicula blastodermica). Its wall is at first com- 

 posed of merely equi-formal cells. But soon, at one point on 

 the wall, arises a disc-shaped thickening, as the cells here 

 increase rapidly, and this thickening is now the foundation 

 of the actual body of the germ or embryo, while the other 

 parts of the germ-bladder serve only for its nutrition. The 

 thickened disc, or foundation of the embryo, soon assumes an 

 oblong, and then a fiddle-shaped form, in consequence of its 

 right and left walls becoming convex (Fig. 7, p. 804). At 

 this stage of development in the first form of their germ or 

 embryo, not only all mammals, including man, but even all 

 vertebrate animals in general — birds, reptiles, amphibious 

 animals, and fishes — can either not be distinguished from 

 one another at all, or only by very unessential differences, 

 such as the arrangement of the egg-coverings. In all the 

 whole body consists of nothing but a quite simple, oblong, 

 oval, or violin-shaped thin disc, which is composed of three 

 closely connected membranes or plates, lying one above 

 another. Each of the three plates or layers of the germ 

 consists simply of cells all exactly like one another; but 

 each layer has a different function in the building up of the 

 vertebrate animal body. Out of the upper or outer germ- 



