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 de 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 differing division of this cell. The cleavage of d** differs 
from that of the other cells of the ventral layer in the following points. 
(1) Its cleavage is very unequal, while the cleavage of the other cells 
is equal. (2) The spindle for the sixth cleavage in de. 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 
*9 and 499 divide unequally, though in all the 
sixth cleavage the cells c 
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 de that 
might oceasion 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 de 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 
ventrad as its anterior surface. The cell is now central, and surrounded 
