Studies on the Life History of Protozoa. 427 



hows that, despite all efforts to stimulate, the race rapidly 

 weakened and ultimately died. 



I. Experiments with Extracts. 



a. Beef Extract. During the last autumnal period, beef extract 

 was used at different times in the same way that it had been used 

 successfully during previous periods of depression, and for the same 

 length of time, twenty-four hours. The organisms were immersed 

 in the fluid full strength in the majority of cases, but experiments 

 with the half strength were also made. The failure of the beef 

 treatment in June, 1902, has already been described (Studies III) 

 and I shall consider in this place only the experiments subsequent 

 to that time. In general it may be stated that beginning with* 

 the treatment in May, the effect of the beef extract was nil. On 

 the 19th of June A3 and A4 were immersed as usual and for the 

 same length of time. Both of them died before the 27th. Again 

 on the 22d Ai and A2 were treated, and both of these died on 

 the 26th. Similar results were obtained in all later experiments, 

 as shown by the following resume: 



Both died before the 15th. 



Ai and A2 died on 25th. A5 on 



the 26th. 

 Both died before the 19th. 

 Both died before the 13th. 

 Died on the 27th. 

 Died on the 29th. 

 Died December 2. 



The short time in beef may not have been long enough to make 

 the change beneficial; with this in mind I kept the last few 

 individuals in for three days (Nov. 26, 27, 28). Not one of them 

 divided more than once and all died within a week. Beef extract, 

 therefore, had lost its potency as a rejuvenating medium. 



The effect of beef extract upon the body structures was to 

 increase the number of gastric vacuoles; while, in some cases, the 

 micronuclei were caused to divide (Figs. 5 and 6). Even in May, 

 1902, there was an indication of the endoplasmic concentration 



