26 EXCITORS OF CELL-DIVISION 



neutralising the alkali in the jelly. In all these 

 experiments 1 c.c. of a 1-per-cent solution of 

 sodium bicarbonate is present in the 10 c.c. of jelly 

 to ensure the correct diffusion of the substance 

 into the cells, and, since no results can be obtained 

 unless this diffusion is arranged for the <( Coeffi- 

 cient of Diffusion" of the cells, it was realised 

 that the neutralisation of the alkali by creatinine 

 might have rendered fallacious our former experi- 

 ments with that substance. Creatinine was there- 

 fore retried with four more units of alkali (1*4 c.c. 

 in all), when it proved to be an auxetic by itself 

 as powerful as creatine. 



Dr. Titherley then suggested that there is a 

 portion of the molecule, the amidine grouping 

 (N C = N), common to creatine, creatinine, and 

 xanthine, and he supplied us with a series of sub- 

 stances, some of which contained this grouping 

 and others which did not. We tried them on the 

 cells haphazardly, but we were not informed of the 

 exact nature of the several substances until after 

 the experiments were complete. 



Out of the series, hippuric acid, ethylamine, 

 ethylenediamine, diethylamine, and glycine ester 

 were not effective, and they do not contain the 

 amidine grouping. But 



Natural : guanidine . . . (4 c.c., 1 per cent) 

 Artificial : benzamidine . . . ( ) 



theobromine . . . ( ) 



acetamidine . . . (3 c.c. ) 



,, caffeine . . . . (4 c.c. ) 



theophylline ...(,, ) 



were all powerful excitors of cell-division like 

 creatine and creatinine. And the striking point 



