JENNINGS : DEVELOPMENT OF ASPLANCHNA HEKRICKII. 



41 



the factors mentioned. Thus, Figure 56 (Plate 7) shows that the cell 

 b^-^ is dividing before the cells c®*^ and b^-'^, though the cells are appar- 

 ently of the same size, and from the sequence of preceding cleavages 

 the cell c^-^ would be expected to divide first. However, such variations 

 mat/ 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 d^-^). 



The cleavage of a single typical quadrant up to this time is shown iu 

 the annexed diagram (Diagram I.). 



Diagram I. 



Diagram of quadrant A, 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 (n, 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 <P-^). 



(.3) All the cleavages are equal except in the dorsal (fifth) layer, and 

 in d^-\ c^-\ and d^-\ 



(\) There is a tendency for the largest cells to cleave fastest. The 

 minute cell d^-'^ does not cleave at all. 



(o) The cell rf®-^ cleaves in such a manner as to form a marked ex 

 ception to the method followed by the other cells. Its cleavage is very 



