Studies on the Life History of Protozoa. 425 



successful as in the previous periods and the organisms were much 

 less vigorous than at similar periods in previous recoveries. The 

 division rate, furthermore, slowly fell from the relatively high 

 point in August, and gradually decreased during the fall months 

 until the A series died on the 19th of December. The B series 

 had succumbed in the 570th generation, in June, before the right 

 stimulus was found. Except for the slowness of divisions the 

 organisms appeared perfectly healthy during the summer and fall 

 of 1902, although microscopical study of preparations made during 

 this period showed characteristic changes in the protoplasmic 

 structure (see Figs. 18 to 21). The organisms were plump and 

 moved freely about the slide, responding with customary vigor to 

 stimuli of diflperent kinds. Every precaution was taken during 

 this period to invigorate the race and every experiment that my 

 ingenuity could devise wks executed; some appeared to give a 

 temporary improvement but none was permanent, and the last 

 individual of the A series finally died after 23 months of continued 

 daily observation, without, however, any morphological evidence 

 of general senility. (See Diagram I.) 



II. UNSUCCESSFUL ATTEMPTS TO REJUVENATE THE A SERIES 

 IN THE FINAL PERIOD OF DEPRESSION. 



Artificial rejuvenation of the A series was successfully accom- 

 plished three times. The experiments and results have been 

 described in other places, and it will be sufficient here to merely 

 point out that after considerable experiment, beef extract was 

 successful in the first two cases and pancreas and brain extract in 

 the third, the result being due, probably, to the change in salt 

 contents of the medium. The approaching end of the series was 

 indicated some time in advance by the reduced division rate 

 during the fall of 1902, and efforts were continuously made to 

 rejuvenate them during this period. For these experiments all of 

 the stock of the regular series was maintained, and the number of 

 lines under observation frequently ran up to twelve or more. 

 The results of all experiments were tabulated and the effects of 

 the stimuli used were noted for comparison with the regular 

 series. The general result may be seen upon the diagram which 



