EFFECTS OF CHANGES OF MEDIUM. 187 



In the 245th generation the increase over the control fell to 2. 

 divisions per line above the control and maintained this same 

 variation when stimulated in the 3OOth generation. Thus in 

 every case the sodium phosphate when a part of the normal 

 medium caused an increase in the vitality of the protoplasm 

 but the amount of the increase varied, being greatest in the 

 height of maturity and less in the period of youth and least in 

 the commencement of old age. 



CONCLUSIONS AND SUMMARY. 



The foregoing experiments seem to indicate a variation in the 

 response of Uroleptus mobilis to a single treatment with salts or 

 to a constant addition of salts to the normal medium. Di- 

 potassium and di-sodium phosphate when used as an initial dose 

 and di-potassium phosphate when added daily to the normal 

 medium cause a distinct depression of the vitality of the young 

 series, in general a stimulation of the more mature individuals, 

 and a slight depression as the protoplasm increases in age. These 

 results agree in the main with those from the beef-flour treatment. 

 If the hydrogen ion concentration of the medium falls much 

 below that of the control, the depressing effects are more marked 

 in both the young and old series and the stimulating effect on 

 the immediately mature individuals is almost obliterated. The 

 increase in acidity may not have been the only cause of the 

 depression as Hartmann (6) found the method of distillation of 

 the water used in his experiments to be an important factor in 

 maintaining a normal culture of Eudorina elegans. When 

 sodium phosphate is used as a constant stimulation there is a 

 general increase in the division rate of all ages, but that exhibited 

 by the young and ageing protoplasm is markedly less than that 

 seen in the intermediate lines. The sodium must increase the 

 permeability of the cells but if this were the whole explanation 

 we should expect to have approximately the same increase in 

 division rate for all the individuals. There seems to be a weaken- 

 ing of the power of adjustment in both the young and old series, 

 in the former possibly the derived organization has not yet 

 become established and in the latter there is possibly an increasing 

 stability or a loss in lability of the protoplasm with advancing age. 



