EARLY DEVELOPMENT OF THE FROG. 



53 



the micromeres very noticeably divide more rapidly than the macromeres 

 with the result that the former become more numerous than the latter 

 (Fig. 28, B, C, D, E, F, G). It is often stated that the rate of cleavage is 

 directly proportional to the amount of cytoplasm and inversely proportional 

 to the amount of yolk. 



B 



H 



FIG. 28. Cleavage of the frog's egg. Morgan. 



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

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



Returning for a moment to the first four blastomeres, the inner edge of 

 each does not quite make contact with its neighbor, and so a minute space 

 is left where the first two cleavage planes intersect. This rounding of the 

 corners is probably due to the tendency for each cell to assume spherical 

 form, which is the natural consequence of its semifluid nature and surface 

 tension. When the third cleavage planes cut the first two at right angles 

 somewhat above the equator, producing eight cells, the inner corners of 



