The Development of the Frog 21 



eventually disappears and forms no part of the 

 adult structure. 



After the thirty-two-cell stage, a series of 

 concentric segmentation planes are formed, 

 dividing the blastomeres into several layers of 

 cells. By the continuation of this process of 

 cell-division the egg is eventually divided into 

 several hundred cells (Figs, i, 6), those of 

 the dark pole being much smaller, more sharply 

 defined, and more numerous than those of the 

 light pole. As is seen in the figures, the seg- 

 mentation cavity lies nearer the dark pole, so 

 that its roof is composed of a few layers of 

 small, dark cells, while its floor is made up 

 of many layers of ill-defined, yolk-filled cells. 

 There is, however, no sharp dividing line be- 

 tween the pigmented and unpigmented cells, 

 any more than there was between the dark 

 and light poles of the unsegmented egg. 



Formation of the Germ-Layers 



The egg, at the close of segmentation, has 

 been converted into a hollow sphere, with the 

 cavity nearer the upper, or dark pole (Fig. 6). 

 The cells of the dark hemisphere are arranged 

 in two more or less distinct layers, while the 

 large, unpigmented cells have no such regular 



