112 THE SMALLEST LIVING THINGS 



Although Weismann's conclusion, with some modifications, 

 is undoubtedly correct so far as some types of protozoa are 

 concerned, it is not correct for the majority of protozoa. Pro- 

 toplasmic differentiation in some types may be carried through 

 continued metabolism to a point where recovery through house- 

 cleaning by division is impossible. Metabolic activities gradu- 

 ally weaken, divisions become more infrequent, and finally the 

 organisms lose the power to divide at all. Such last individuals 

 of a race may struggle along for a few days or even weeks ; in 

 one case an individual survived for three months without divi- 

 sion, but all finally die a "natural death." Without going into 

 the long history of isolation culture experiments, which were 

 started by Maupas in 1887 in an attempt to disprove Weismann's 

 assertion in regard to "immortality" in protozoa, I will cite one 

 recent experiment which has a direct bearing on the problem. 



Testing for Vitality 



Uroleptus mobilis is a ciliated protozoan of relatively sim- 

 ple structure which lives perfectly well when isolated in a drop 

 of hay infusion and flour (Fig. 50, page 84). A single ex-con- 

 jugant was isolated in November, 1917, and its protoplasm was 

 observed and studied daily for a period of more than ten years. 

 At the outset it divided approximately seventeen times in ten 

 days, and the division rate, by ten-day periods, was adopted as 

 a measure of vitality. If we plot the numbers of divisions in 

 successive ten-day periods we obtain a curve of vitality, such as 

 that shown in Figure 64, which is a composite graph of the life 

 histories of twenty-three ex-conjugants. The initial vitality was 

 a little more than fifteen divisions in ten days; after five months it 

 had steadily fallen to ten divisions in ten days, after eight months 

 to three divisions, and the organisms were all dead at the age 

 of thirteen months. This history indicates a slowly changing 

 protoplasmic organization in which the vital activities were car- 

 ried on with increasing difficulty until they stopped altogether. 

 It is evident that cell division does not effect a complete reorgani- 

 zation in Uroleptus and that the daughter cells, after each 

 division, start out with a potential vitality lower than that of 

 the parent cell. 



It must be remembered that in such an experiment as here 



