556 H, S. Jennings and George T. Hargitt. 



but as soon as they are fulfilled, the heritable size remains con- 

 stant. It was therefore important to determine whether these 

 conditions had been fulfilled in the cases cited by Popoff. Analy- 

 sis seems to show that they had not. If in this analysis any 

 essential points have been overlooked, Popoff will be able to set 

 the matter right. 



This question should be readily resolvable by experiments in 

 cutting Stentor or Spirostomum into pieces. Lillie ('96) showed 

 that pieces of Stentor one-twenty-seventh the entire animal 

 will regenerate. It should not be difficult to get such small 

 pieces containing a proportionately small fragment of the nucleus 

 and to determine by culture whether the race produced by such 

 a small specimen remains small, or whether it finally returns to 

 the normal size. Popoff's work shows that the volume of the 

 cytoplasm can be regulated to accommodate itself to the size of 

 the nucleus. His conception is that new races are produced 

 when both nucleus and cytoplasm are smaller (or larger) than 

 usual; since it is the proportionality that is restored, not the ab- 

 solute size. This can be subjected to rigid experimental test by 

 such work as we have mentioned. 



In any case, of course, the method of action set forth by Popoff, 

 even if it turns out to be correct, would account only for the 

 differences in size. The physiological differences set forth in this 

 paper, and the differences in form, in structure of micro-nuclei, 

 and the like — the origin of these requires other methods of action. 

 The problem will be dealt with in the further investigations on 

 Paramecium. 



10. List of the races or lines dealt with in this paper, with 



their characteristics 



In conclusion it will be well to give here in brief, for reference, 

 a tabulation of the races or lines mentioned in this paper; this 

 will further serve as a sort of summary indicating the sort of 

 racial diversities one is likely to meet in working with Protozoa. 

 Each race or line is derived from a single individual. The desig- 

 nations used are the same as those employed by Jennings in 1908, 

 and in the present paper. 



