STUDY OF CELL MECHANICS 475 



conclusive evidence that these changes (at least the first two) 

 in monaster eggs are due to another cause. 



With the elimination of the aster from our problem, we still 

 have in the nucleus, and in the cytoplasm, two possible sources 

 for the behavior of protoplasm in monaster eggs. I have been 

 unable to devise experiments which would give decisive evidence 

 here. On the whole the mass of evidence is in favor of the 

 nucleus, though we are utterly in the dark as to how it works. 

 In addition to the evidence discussed on page 468 in favor of 

 the nucleus, the experiments with phenyl urethane show that 

 when cytoplasmic cleavage does occur, it cuts through the re- 

 gion where the chromosome spindle lay. The position of the 

 chromosomes seems to play a part here. 



As additional evidence we may point out that cytoplasmic 

 cleavage rarely or never occurs in the absence of chromatin 

 from the spindle. This is nowhere shown with greater clear- 

 ness than in Boveri's paper "Zur Physiologie der Kern-und 

 Zellteilung" ('97). In cells with tetrasters, cleavage only 

 took place between poles in which chromosomes were present. 

 Although shallow cleavage furrows might appear between asters 

 where no chromatin was present, lasting division was never 

 seen. 



While at first sight the elaboration of Loeb's suggestion 

 which Robertson ('08) has given seems plausible enough, there 

 are difficulties to be met which are not easy to overcome. As 

 is well known, Robertson suggested that some substance (cholin) 

 diffused through the cell cytoplasm on the disappearance of 

 the nuclear wall, and when this substance reached the surface 

 of the egg, it formed soaps which lowered the surface tension of 

 this part of the egg and hence brought about division. 7 If 

 the swelling of the ectoplasmic layer and the phenomena fol- 

 lowing it were the result of simple diffusion, it seems probable 

 that it would appear in all eggs about the same time. Far from 

 this, as I have repeatedly pointed out, the period when the 



7 Concerning Robertson's contention that cell division is brought about by 

 a lowering of the surface tension at the equator of the egg, see footnote 4, page 

 467. 



