CLEAVAGE. 



43 



elements lying, as they always do, within the protoplasmic portion of the cell, 

 determine the next cleavage plane which is horizontal and lies nearer the proto- 

 plasmic ends of the cells. The result is that the third cleavage gives rise to 

 eight cells, four of which are small protoplasmic cells lying above the line of 

 cleavage, while the other four are large yolk-containing cells which lie below 

 the line of cleavage (Fig. 27, A). This distinction between protoplasmic cells 



B 



D 



H 



I 



FIG. 27. Cleavage of the frog's egg. Morgan. 



A, Eight-cell stage; B, beginning of sixteen-cell stage; C, thirty-two-cell stage; D, forty-eight-cell 

 stage (more regular than usual); E, F, G, later stages; H, I, formation of blastopore. 



and yolk cells not only persists but tends to become more and more marked as 

 segmentation proceeds, and it soon becomes evident that the cells unencum- 

 bered by yolk have a tendency to segment more rapidly than do their yolk- 

 laden brethren (Fig. 27, C, D, E, F and G). Thus, while the fourth cleavage 

 is vertical in both types of cells, giving rise to eight upper protoplasmic cells 

 and the same number of lower yolk cells, this uniformity of number persists 



