228 THE CYCLE OF CELL-DIVISION 



considerable time, many points connected with the proc- 

 ess of inducing divisions were learnt. We had three 

 factors to deal with, viz. polychrome dye, atropine, and 

 extract consisting of the soluble remains of dead haemal 

 gland of 100 per cent. It was found that lympho- 

 cytes would not make any attempt whatever to divide 

 unless they absorbed some of the polychrome stain. As 

 the stain passed into the cells, it stained first their 

 chromosome-granules and their nucleolus-centrosomes. 

 Like polynuclear leucocytes, lymphocytes do not appear 

 to surfer much harm to their Jives while their granules 

 are stained, but as soon as their nucleolus-centrosomes 

 are reached by the dye death occurs. Mitosis takes 

 place about the time when the granules are staining, 

 and therefore the rapidity of the onset of mitosis depends 

 on the rapidity of the diffusion of the dye into the cells. 

 It is thus evident that the gradual diffusion of the 

 stain first causes mitosis and then death because it 

 kills the cells by combining with and staining the 

 nucleolus-centrosome. The rapidity of the diffusion 

 of the stain is increased by concentrating it, by the 

 presence of alkalies, or by heat. These factors also 

 hasten death and they likewise hasten cell-division. 

 With regard to the factor heat, however, we must add 

 the qualification that no lymphocyte will divide below 

 a temperature of 30 C. or above about 40 C., and for 

 this reason we have employed a temperature of 37 C. 

 throughout these experiments for inducing division. 



Now, mitosis is a process which occupies a certain 

 amount of time. If the diffusion of the stain is 

 very slow, the time taken by the act of mitosis is 



