42 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



unequal, while all the other cells in the ventral layer cleave equally. 

 Also, the spindle does not lie in the same direction as in the other cells 

 of this layer. 



The exceptional nature of the division in d^-^ evidently demands 

 explanation. The regularity of the cleavage in the other cells of the 

 egg is such that some special condition must be correlated with the 

 markedly dift'oring division of this cell. The cleavage of d^-^ difiers 

 from that of the other cells of the ventral layer in the following points. 

 (1) Its cleavage is very imequal, while the cleavage of the other cells 

 is equal. (2) The spindle for the sixth cleavage in d^-^ lies in almost 

 exactly the same position as did the spindle for the preceding cleavage, 

 whereas in the other cells of the ventral layer the spindle for the sixth 

 cleavage is at right angles to the position of the spindle for the fifth 

 cleavage. 



As to the first point, — that of unequal division, — no special cor- 

 relating factor for this particular case seems necessary, since in many 

 cases in this and preceding cleavages the cells varied as to the equality 

 of the division, though alike in other respects. Thus, in the fifth 

 cleavage, the three ventral cells of the D quadrant cleft unequally, 

 though in all the other quadrants the division was equal ; and in this 

 sixth cleavage the cells c®'® and d^-^ divide unequally, though in all the 

 other cells of the same layer the division is equal. 



But the second point is totally exceptional in the cleavage up to this 

 time. The axial relations of the cells appear to be so distinct and con- 

 stant, and there is such uniformity in the positions of spindles at a 

 given cleavage among cells of similar origin and relative position, that 

 one must look for some other marked difference in the cell d^-^ that 

 might occasion this change of axis. 



In what respects does the cell d^-^ differ from the other cells of the 

 egg'? (1) In its greater size ; (2) in its position. 



(1) The greater size evidently has nothing to do with the different 

 direction of cleavage, since the same disparity in size was present in 

 earlier cleavages, yet this cell divided in the same direction as did the 

 other ventral cells. 



(2) The change of position is such as to bring about a fundamental 

 change in the relations of the cell to the egg as a whole and to the 

 other cells. Previously the cell d^-^ formed the posterior cell of the 

 ventral layer. At the time of the sixth cleavage the cell has moved 

 toward the interior of the egg and its posterior surface is covered as far 

 veutrad as its anterior surface. The cell is now central, and surrounded 



