THE MITOTIC CYCLE 



used in these stratification experiments vary from 490 x^ for 15 minutes 

 (KosTOFF^"^) to 12,065 X g for 20 seconds {Arbacia, Fry and Parks^*^^),* 

 whereas Beams and King^*'' found that as much as 150,000 X g for 

 10 minutes was necessary to distort the contents of the cells of chick 

 embryos; under these conditions both dividing and resting cells were 

 equally affected. In preliminary experiments in which the magnetic 

 particle method of Crick and Hughes^*'^ was applied to chick cells in 

 culture, there seemed to be no regular differences in protoplasmic 

 consistency between cells in interphase and mitosis. 



It has long been customary to relate these and other changes in 

 living protoplasm to reversible gelation; in this way some of the general 

 concepts of colloid chemistry have been applied to living material both 

 in relation to the dividing cell and to protoplasmic activity such as 

 amoeboid movement. The physiology of mitosis has been discussed in 

 these terms by Spek^"^ for example, and by Bujard.^^" The study of 

 the effect on these changes of applied hydrostatic pressures by Mars- 

 land, Brown and Pease has lifted this subject from a purely descriptive 

 phase into one of experimental analysis, and has enabled these workers 

 for instance to distinguish between different types of sol-gel equilibria, 

 according to whether volume changes are involved therein. The effect 

 of applied pressure on such equilibria can be deduced from the principle 

 of Le Chatelier. (General reviews of this field are given by Mars- 



LAND^ll 31ia ) 



Pressures of the order of thousands of atmospheres reversibly inhibit 

 such biological activities as amoeboid movement, protoplasmic stream- 

 ing in plant cells and cell cleavage. As far as can be judged from the 

 available evidence, however, other features of the mitotic process are 

 not uniformly affected to the same extent. Marsland^^^ states that if 

 the first cleavage is suppressed in the egg o^ Paracentrotus, two nuclei 

 are afterwards found within the undivided &gg\ during the next cleavage 

 period such an c.gg divides into four blastomeres if the pressure is 

 released. There must thus be some anaphase movement of the chromo- 

 somes under pressure although cleavage is prevented. Pease^^^ found 

 that the fibrous structure of the asters and spindle disappears in eggs 

 of Urechis when fixed after the application of 210 atmospheres, but at 

 this pressure, the chromosomes still move in anaphase, though at a 

 much retarded rate. They often then coalesce into vesicles and Pease 

 suggested that this effect on the chromosomes is due to liquefaction of 

 the sheath. The same author (Pease^^*) later found that the spindle of 

 the pollen mother cells of Tradescantia can be reversibly solated by 

 pressure. The salivary chromosomes of the Drosophila larva are appar- 

 ently insensitive to hydrostatic pressure (Pease and Regnery^^^). 



* The changes in consistency of the gelated cortex of the sea-urchin egg will be discussed 

 in the pages that follow. 



138 



