JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 41 
tho factors mentioned. "Thus, Figure 56 (Plate 7) shows that tho cell 
56. wis dividing before the cells cèt and bee, though the cells are appar- 
ently of the same size, and from the sequence of preceding cleavages 
the cell ce. would be expected to divide first. However, such variations 
may be correlated with differences in the size of the cells, since it is 
impossible to calculate precisely the volume of cells which have such 
irregular forms, and are subjected to varying conditions with the chan- 
ging positions of the surrounding cells. 
Certain general facts appear from the preceding discussion of the 
sixth cleavage. (Compare the table of this cleavage, on page 39.) 
(1) Every cell of any quadrant cleaves with its spindle in the same 
direction as the corresponding cell of any other quadrant (except the 
large interior cell de.). 
The cleavage of a single typical quadrant up to this time is shown in 
the annexed diagram (Diagram I.). 
Dracram I. 
Diagram of quadrant 4, B, or C in 
the seventh generation. Only the second 
exponent designating the cells appears in 
the diagram, the first being in all cases 7. 
Thus, the cell labelled 5 represents (4, b, 
or c)^5, The arrows connect cells of 
common origin, and show the direction 
of the spindles at the preceding division. 
R signifies right; L, left; D, dorsal; 
V, ventral; according to the plan of 
orientation explained at page 14. 
(2) All the cells in any layer (series of cells occupying the same 
relative position between the dorsal and ventral poles of the egg) cleave 
with spindles in the same direction (except 0 
(3) All the eleavages are equal except in the dorsal (fifth) layer, and 
In and Urn 
(4) There is a tendeney for the largest cells to cleave fastest. The 
minute cell d*? does not cleave at all. 
(5) The cell de cleaves in such a manner as to form a marked ex 
ception to the method followed by the other cells. Its cleavage is very 
