SEGMENTATION OF THE EGG. 119 



situated nearer the upper than the lower pole of the egg, and is 

 filled with fluid. 



At the close of segmentation the egg has the structure shown 

 in section in Fig. 23. 



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. 



The distinction between upper and lower cells is however not 

 an absolute one, a ring of 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. 



E 



Fig. 23. Vertical section through a frog's egg at the close of segmen- 

 tation. X 28. 



E, epiblast : SC, segmentation cavity : Y, lower layer or yolk cells. 



The process of segmentation is as mentioned above, simply 

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

 parts of the egg segment are to be regarded as due to the 

 retarding influence of the granules of food-yolk, which, being 

 themselves inert, must hinder the activity of the protoplasm in 

 which they are imbedded. These granules of food-yolk are more 



