THE EARLY DEVELOPMENT OF THE FROG 93 



fifth cleavage the early, and brief, synchronism of the cleavages 

 has become lost, the small upper cells dividing more rapidly 

 than the large lower cells. Comparatively few eggs remain 

 regular up to this stage, for eggs which were regular at eight 

 cells usually become irregular at sixteen or thirty-two cells, and 

 after that all regularity is lost (Fig. 30). 



Turning back now to notice some of the internal arrangements 

 of the cells during cleavage, we find that when four cells are 

 cut into eight these all round off somewhat internally, as in 

 Amphioxus, forming a small space among them, which is the 

 beginning of the segmentation cavity or blastoccel, and which 

 from the position of the third cleavage has from the first an 

 eccentric position toward the animal pole. During subse- 

 quent cleavages the blastocoel enlarges but always retains this 

 eccentric location (Fig. 31, A}. 



After about thirty-two cells are formed not all of the sub- 

 sequent cleavages are visible on the surface, for these early 

 divisions, all passing through the axis of the egg, have formed 

 cells elongated in a radial direction, and in some of these the 

 cleavage spindles tend to take up a similar position and the 

 resulting division occurs parallel with the surface of the mass, 

 forming a central cell bordering the segmentation cavity and 

 a superficial cell visible externally (Fig. 31, B). There is no 

 period at which such a delaminating cleavage occurs through- 

 out the cell group, but scattered cells show this arrangement, 

 first among the cells of the upper hemisphere and then later 

 in the lower cells, which are divided quite unequally in this 

 way. Many of these interior cells are formed by cleavages 

 that are not exactly tangential but considerably oblique to the 

 surface. By the time there are sixty-four or one hundred and 

 twenty-eight cells, approximately one-fourth of the cells are 

 interior, and line the blastocu'l. so that at this stage the wall 

 of the blastocoel is two or more cells thick. 



4. The Blastula 



We may assume that the arrangement of the cells forming the 

 wall of the blastoccel as a more or less definite epithelium, 



