72 FUNDAMENTALS OF CYTOLOGY 



becomes increasingly probable that repulsions of some sort do play a role, 

 llioush forces of attraction are more difficult to demonstrate. If the 

 chromosomes, which carry a negative charge, lie in a spindle with positive 

 poles, the combination of forces could result in movement. The same 

 result could follow if between all parts there were repulsions and these 

 varied in relative intensity during the nuclear cycle. Attraction of 

 centrioles for chromosomes is sometimes strongly suggested in the pro- 

 phase when they move along together on opposite sides of the nuclear 

 membrane, but its importance here or in anaphase is still uncentain. 



Anaphasic movement evidently depends not only upon the spindle 

 mechanism, but also upon changes going on in the chromosomes them- 

 selves. This is indicated by special cases, including hybrids, in which 

 metaphasic arrangement and anaphasic movement occur only after 

 chromosome doubleness, particularly at the kinetochore, has developed 

 to a certain stage, even though the spindle is active earlier. It has also 

 been thought that a special sheath-like differentiation at the chromosome 

 surface undergoes local and progressive viscosity changes in such a way 

 as to result in endwise movement. In spermatocytes of certain fungus 

 flies (Sciara) the achromatic figure is monopolar, and 10 single chromo- 

 somes lie scattered mthin it. Of the 10, 6 regularly go toward the single 

 pole and 4 away from it, although all have spindle attachments facing the 

 pole. Furthermore, the 4 which pass away from the pole and are 

 eventually extruded from the cell are always the same 4 out of the set 

 of 5 originally contributed by the male parent. This indicates clearly 

 that the reactions of a chromosome in the spindle are determined in part 

 by specific constitutional features of the chromosome itself. 



All the foregoing considerations lead to the conclusion that the 

 behavior of chromosomes at anaphase and other phases of mitosis is 

 brought about by a nicely correlated combination of forces, even though 

 it is not yet possible to name them all or to estimate their relative impor- 

 tance. To a certain extent we know well ivhat occurs: the equational 

 separation of certain key materials of the nucleus. We also know why 

 this occurs, in the sense that we can state its biological significance with 

 respect to ontogeny and heredity. How it occurs we know least of all. 

 Cytologists, now that they may take advantage of new developments in 

 physical chemistry, are ever more confident that an adequate explanation 

 of chromosome movement can some day be reached. 



Time Occupied by Cell Division. — The amount of time required for a 

 somatic division to be carried through and the rapidity with which 

 divisions succeed one another are found to vary in different tissues and 

 organisms. They also vary with temperature. Mitosis in the Trades- 

 cantia stamen hair occupies about 30 minutes at 45°C., 75 minutes at 25°, 

 and 135 minutes at 10°. In dividing stigma hair cells of Arrhenatherum 



