THE DIVISION OF THE PliOTOPLAST 71 



FURTHER ASPECTS OF CELL DIVISION 



Causes of Anaphasic Chromosome Movement. — To anyone who 

 follows the nucleus through all the visible changes comprising one 

 mitotic cycle, it is plain that the problem of explaining this cycle in terms 

 of physics and chemistry is one of extraordinary complexity. It has 

 nevertheless been hoped that an understanding of at least one phase of the 

 process, anaphasic chromosome movement, might soon be reached. A 

 full account of attempts to solve this part of the problem would occupy 

 many pages, but the conclusion would be a brief one, viz., that no satis- 

 factory solution has yet been foimd. At the same time it should be 

 helpful to enumerate some of the principal observations and hypotheses 

 that promise to contribute to an eventual explanation. 



The early theory that spindle fibers attached to the chromosomes 

 simply contract and drag the chromosomes apart has not fared well in 

 the light of subsequent work. Recently, however, an oscillatory inde- 

 pendent movement of the several chromosomes at metaphase observed 

 in living cells has brought the suggestion that localized alterations in 

 viscosity (gel-sol changes) in the immediate neighborhood of the chromo- 

 somes play a role in their later movement, for gelation, as stated in the 

 previous section, is known to be accompanied by contraction in many 

 nonliving colloidal systems. 



Appearances near the kinetochores, in particular the formation of 

 "tractile fibers" and small projections on the chromosomes w^here 

 movement begins, strongly suggest a slow streaming of the viscous 

 materials; moreover, the aster when present is known to have streams 

 flowing toward the poles. Despite these appearances it has not j'et been 

 possible to demonstrate that diffusion streams in the spindle substance 

 are a major factor in chromosome movement, and if this were demon- 

 strated the streaming would still have to be explained. 



Elongation of the spindle, which is sometimes observed and can be 

 experimentally modified, has been cited as a factor in chromosome 

 movement. In one prominent hypothesis the initial separation of the 

 chromatids was attributed to an action of the tractile fiber mechanism, 

 subsequent movement poleward being due to spindle elongation in the 

 region between the two lots of chromatids attached to it. This inter- 

 pretation, too, has met obstacles: elongation may not occur; experimental 

 alterations of spindle viscosity may not produce the expected effects 

 on movement; chromosomes in some organisms, notably in hybrids, may 

 not all move poleward even though they occupy the equatorial plane 

 together at metaphase. 



Forces of electrical repulsion and attraction have long been looked 

 upon as factors of special importance. As investigations continue it 



