SEGMENTATION OF THE EGG 111 



It is a hollow ball with its walls composed of three or four 

 layers of cells, and of very unequal thickness, owing to the 

 segmentation cavity lying in the upper half of the egg. The 

 cells of the upper half are small, fairly uniform in size, and 

 regularly arranged, while those of the lower half are larger, 

 and more irregular both in shape and size. The superficial 

 cells of the upper half are deeply pigmented, while the cells 

 of the lower half are almost colourless. 



FIG. 22. Vertical section through a frog's egg at the close of seg- 

 mentation, x 28. 



E, cpiblast ; SC, segmentation cavity ; Y , lower layer or yolk cells. 



The distinction between upper and lower cells is however 

 not an absolute one, a ring of numerous cells more or less 

 intermediate in size, shape, and depth of pigmentation, 

 occurring round the equator of the egg at the junction of 

 its upper and lower halves. These intermediate cells (Fig. 

 22A, 1C) take an important part in the formation of the frog's 

 tissues. 



The process of segmentation is, as mentioned above, simply 

 one of cell-division; and the unequal rates at which the 



