142 SENESCENCE AND REJUVENESCENCE 



dividing pairs and the single animals belong to different races which 

 differ in susceptibility cannot of course be excluded in individual 

 cases except in pure line cultures, but the uniformity of the results 

 obtained with large numbers of individuals and in repeated tests 

 render this possibility negligible. 



But the susceptibility is highest after fission is completed. In 

 all cases th^ smaller individuals are in general very clearly more 

 susceptible than the larger. This difference is not a matter of 

 size or of the relation between surface and volume, for the cilia and 

 the whole body-surface show it. The cilia and ectoplasm of the 

 larger animals are in general much less susceptible to a given con- 

 centration of cyanide than those of the smaller animals. As death 

 and disintegration proceed in a lot consisting of hundreds or thou- 

 sands of individuals, it soon becomes very evident that the smaller 

 animals are dying earlier than the larger. In a culture of Colpidium, 

 for example, where division was proceeding very rapidly, animals 

 below a certain size were more than twice as numerous as those 

 above this size, but after deaths began to occur in cyanide, the 

 smaller animals became less than half as numerous as the larger, 

 and still later only about one small to five large was found ahve. 

 Similar results were obtained with the other species. In a Stentor 

 culture where divisions were occurring only in the animals of 

 medium size or above, the susceptibihty of the animals below 

 medium size was much greater than that of the larger animals. 

 Some of the smaller animals in these cultures may conceivably 

 have belonged to small races possessing a greater susceptibihty at 

 all stages than the large, but as the culture was increasing rapidly 

 in numbers, most of them were certainly the products of recent 

 fission. 



These data are in complete agreement with those obtained 

 from the study of the flatworms and indicate very clearly that an 

 increase in rate of metabolism is associated with the process of 

 fission in the ciliate infusoria and that the rate of metaboUsm is 

 highest soon after fission. In other words, after fission the 

 animals are physiologically younger than before fission, and in 

 the interval between two fissions they undergo some degree of 

 senescence. 



