48 



CELL-DIVISION AND GROWTH 



II. i 



other factors must intervene, just as other factors intervene in 

 a mass of soap-bubbles their size and initial arrangement 

 in the determination of the actual pattern. These other factors 

 are the direction of cell-, that is of nuclear, division, and the 

 magnitude of the cells ; and these, as we have seen, in turn 

 depend upon the relation between the nucleus and the cytoplasm 

 with its included yolk. Thus it is the direction of the spindles 

 which determines whether the micromeres of the first quartette 

 shall be given off laeotropically or dexiotropically ; the direction 

 of division, oblique to the egg-axis, again determines that the 

 micromeres shall alternate with the macromeres and not be 

 superimposed upon them ; the size of the cells and the direction 



B 



FIG. 25. Mitotic division with elongation of the cell-body in a proto- 

 zoon,Acanthocystis aculeata. (After Schaudinn, from Korschelt and Heider.) 



of division may determine the position of the polar furrow, 

 while the rate of division will also not be without effect, since 

 the whole arrangement at any stage depends in part on the 

 disposition at the stage before. 



There is one other point that is worthy of notice. The mitotic 

 spindle possesses considerable rigidity, and is able as it elongates 

 to materially alter the shape of the cell. This may be seen in 

 many cases in Annelid, Molluscan and other eggs the division of 

 the first micromeres in Nereis is an instance and in the Protozoa 

 (Fig. 25). Another interesting case is the Rotifer Asplanckna, 

 where, preparatory to the fourth division, the shortest axis of 

 the cells in which the spindles are placed becomes by the 

 elongation of the spindles the longest. This alteration of shape 

 is itself an important factor in deciding the positions to be taken 

 up by the daughter cells. 



