Variations in Paramecium aurelia. 6i 



later the same author 1 reverses his opinion and based on extensive 

 studies, claims that a single stock, i. e., a pure line derived by 

 fission from a single progenitor gradually differentiates into such 

 hereditarily diverse stocks; so that by selection marked results 

 are produced. His former conception of the genotype was gained 

 by experiments with the highly specialized infusorian species 

 Paramecium caudatum and aurelia, his later ideas by his results 

 with Difflugia corona, an amcebina of simpler cytological structure. 



Difflugia has definite structural characters that can be counted 

 and measured, which are unchanged by growth and environmental 

 conditions, but still are "hereditable, yet variable." This coin- 

 cidence of favorable conditions — besides theoretical considerations 

 — gives a priori more support to views that do not maintain the 

 absolute constancy of the genotype, though they contradict the 

 current conception of the "so-called" pure line. Paramecium is 

 changed in size by its daily divisions and by the influences of the 

 environment, thus complicating genetic studies; Jennings's results 

 have been challenged by Walton 2 and Castle, 3 as statistically 

 considered far from conclusive. The mean length, a constant 

 after his opinion, varies between 114.033, 123.606, 130.120 and 

 144.880 microns for mass cultures, derived from the same animal. 

 A new complication arose after the discovery of endomixis, 4 a 

 dynamic, periodic reorganization process, involving the disinte- 

 gration and absorption of the old macronuclear and micronuclear 

 material without the introduction of foreign nuclear material. 

 It was evident that the discrepancies in Jennings's measurements 

 might be explained, if there are periodically appearing fluctua- 

 tions during the intermictic periods. Further, before the influence 

 of conjugation in Paramecium can be studied, it is necessary to 

 know first the isolated influences of division, clearly shown by 

 Jennings and reinvestigated for the lines I worked with, to elimi- 

 nate the changes produced by environmental conditions and the 

 most difficult process, to find, if endomixis has influences on the 

 variability of the quantities of the line. These are the preparatory 



1 Jennings, H. S., Genetics, i, 407-534. 



2 Walton, L. B., American Naturalist, 49, 642-652. 

 ' Castle, W. E., American Naturalist, 50, 179-183. 



* Woodruff, L. L., and Erdmann, Rh., Jour. Exp. Zobl., 17, 425-516; Erdmann, 

 Rh., and Woodruff, L. L., Jour. Exp. Zool., 20, 59-96. 



