492 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



the differences in size among these four Hnes were very evident to 

 the eye on inspection with the low power of the microscope, and 

 that the difference was clearly present at all periods between the 

 dates when the measurements were made. The measurements 

 merely make precise what is evident to the eye without them. 



Before attempting to determine whether still other lines can be 

 isolated, and particularly whether it is possible to fill the wide gap 

 between the caudatuni group and the aurelia group, another question 

 must be investigated — a question which strikes at the foundation of 

 our conclusions up to this point. This is the question of the relation 

 of these lines of diverse size to conjugation and the life cycle. 



(b) Are the Lines of Different Size Merely Different Stages in the 



Life Cycle? 



Calkins (1906) and others have set forth the fact that Para- 

 mecium and other infusoria show dift'erent dimensions in different 

 stages of the life cycle — the cycle which begins with conjugation, 

 extends over many generations of reproduction by fission, and ends 

 with another conjugation. The question arises, therefore, whether 

 our lines of diverse dimensions are not merely different stages in the 

 life cycle; whether they would not, if brought to the same stage of 

 the cycle, show the same dimensions. This possibility must be 

 investigated before we proceed farther. 



The details of the relation of conjugation and the life cycle to 

 variation, inheritance, etc., are to be dealt with in a separate paper 

 of this series. But since the question which stands at the head of 

 this section is an absolutely fundamental one for the proper inter- 

 pretation of the results of the present paper, it must be dealt with here. 



To answer this question, it is evidently necessary to proceed as 

 follows: Cultures showing epidemics of conjugation must be exam- 

 ined for conjugating pairs of diverse sizes. If such are found, the 

 individuals must be isolated and allowed to multiply, in order to 

 determine whether the progeny retain the diverse sizes characteristic 

 of the parents. If from a conjugating culture we can obtain diverse 

 lines standing all in the same relation to conjugation and the life 

 cycle, then evidently our diverse lines represent something more 



