STUDIES ON LIFE-HISTORY OF PROTOZOA. 199 



generation (June 2jtJi] it looked as though the cultures were 

 about to come to an end. The unusual appearance of the or- 

 ganisms made me think that the trouble was not with the diges- 

 tion of food, but with its assimilation, and with this clue in mind 

 I made extracts of sheep's brain and sheep's pancreas. Three of 

 the six remaining individuals were put into the brain -extract and 

 three into the extract of pancreas. The latter worked best and 

 the 62 ist generation appeared the next day. The former, while 

 giving slower results, produced healthy and normal organisms, 

 and in three days there were more than 30 individuals, and 

 enough to experiment with. After 48 hours' treatment with 

 each of these extracts the organisms of the regular series were 

 transferred to the usual hay-infusion. Since then the division- 

 rate has been fairly constant, and, as stated at the beginning of 

 this paper, they are to-day (July 28th) in the 665th generation. 



The effect of the two extracts was very interesting. The indi- 

 viduals that were treated with pancreas had been in N/25 magne- 

 sium chloride for thirty minutes on the sixth day of May. Those 

 that were treated with the brain-extract had been in N/I25O 

 potassium phosphate for thirty minutes on the eighth of June. 

 The division-rate of the former, notwithstanding the quicker start, 

 soon fell behind that of the latter, and to-day the only ones alive 

 are the descendants of those that had been fed with the brain- 

 extract. 



The interpretation of these facts involves the interpretation of 

 many of the deepest problems of biology. If we could point out 

 their significance, it would mean that we are in a position to 

 throw light upon the ultimate secrets of vitality ; secrets which 

 have taxed the ingenuity of philosophers of all ages, and which 

 lie at the bottom of modern biological research. At the present 

 time the question cannot be answered, and it seems mere pre- 

 sumption to attempt an explanation. Nevertheless, there are 

 sufficient data to warrant a tentative interpretation even though 

 it is only a vague, shadowy impression of what may actually go 

 on in living protoplasm. 



The bacteria in the hay-infusion constitute the normal food of 

 the Paramcecidse. Of these, Bacillis sitbtilis, is, probably, the only 

 one left alive after the infusion is raised to the boiling point, and 



