ACCELERATION OF RATE OF CELL DIVISION. 353 



a detailed statistical treatment of a total of 55,000 cell counts, 

 divided the mitotic process into ten stages whose limits were arti- 

 ficially defined, and determined the actual time of each stage at 

 10, 20, and 30 C. For the period of rapid elongation of the 

 root tips the values for each stage are shown in the tabulated 

 resume. 



Laughlin's observations upon the periodic character of mitosis 

 and its rhythms have already been referred to. 



The factors which govern cell division are not as yet clearly 

 understood. In a matter of such vital character as mitosis and of 

 such complexity it is not to be expected that clear conceptions will 

 be easily forthcoming. One method by which an analysis may be 

 attempted is that of modifying the process by various means. At 

 the present time little in the way of reliable data is at hand to 

 indicate to what extent or by what factors the rate of division may 

 be modified. 



There are many factors that will act to decrease the rate ; in fact, 

 any condition which is unfavorable to the growth and well-being 

 of the organism, or which lowers its general metabolic activity, 

 results in a decreased rate of division in tissues where multiplica- 

 tions are taking place. Some of these factors are definite and 

 measurable in their results, as decreased temperature, while others 

 are intangible and seem to affect the vitality of the entire organism 

 rather than the mitotic mechanism. Because of this latter set of 

 factors it is not profitable to attempt here a study of the causes of 

 a lowered rate of multiplication. 



On the other hand, the factors which can cause an increase in 

 the rate of division are few, and must in the nature of the case 

 work their effects directly upon the mechanism of mitosis. It is 

 within the bounds of hope that their study will throw light upon 



A 



this mechanism as well as upon the matter of differentiation and 

 organization. Since little attention has been given to this matter 

 by cytologists and experimental embryologists, it seems desirable 

 to list the means by which increased division rates have been 

 obtained and to point out at some length the nature of their effects. 

 The list of agencies by which acceleration of the rate of division 

 may be accomplished is not a long one. One might infer that any 



