SEGMENTATION OF THE EGG 109 



completion of the thii'd cleft, a small cavity appears in the 

 centre of the egg, round which the cells are grouped : during 

 the later phases of segmentation this segmentation cavity or 

 Mastocoele, as it is called, increases considerably in size : ' it is 

 from the first 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 structvu'e shown 

 in section in Fig. 22. 



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 



E 



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

 mentation. X 28. 



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



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. 



The process of segmentation is, as mentioned above, simply 



