CH. X] 



MODIFICATION OF CLEAVAGE 



103 



the cleavage-planes that come in at right angles to the cleav- 

 age-spindle must be vertical and at right angles to the plates. 

 The third cleavage-planes will be for the same reason vertical, 

 and even the fourth planes may be so. The number of con- 

 secutive divisions at right angles to the compressing plates must, 

 however, soon reach a limit, because the mass of protoplasm 

 in each cell will soon be thicker vertically than horizontally. 

 When this happens, the next cleavage comes in horizontally or 

 parallel to the plates. 



Hertwig's hypothesis seems, therefore, in harmony with the 

 phenomena of the compressed eggs. Whether it is of general 

 application may be doubted because cases have been recorded 



FIG. 33. Diagrams to show the distribution of nuclei in compressed (A-D) and 

 normal egg (E-H). In the upper series (A-D) the black hemisphere is turned 

 toward the observer ; in the lower series (E-H) the egg is seen from the side and 

 in part from above the black hemisphere. 



where the elongating spindle does not seem to take the direction 

 of the greatest protoplasmic mass. Further, in certain spheri- 

 cal eggs without yolk, all the axes are equal, and some other 

 cause must be present to determine the direction of the spindle. 

 Even in the compressed egg (category 1) the protoplasm must 

 be radially symmetrical. Finally, it is possible that the phe- 

 nomena of the greatest protoplasmic mass and the elongating 

 spindle may be only concomitant and not causal phenomena, for 

 the position assumed by the centrosomes, which come to lie at 



