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THE AMERICAN NATURALIST [Vol. XL VII 



it is the 'composition' of the medium rather than the changes 

 in the medium which is conducive to the unlimited development 

 of this culture without conjugation or artificial stimulation" 

 (15, page 141). 



From this basic investigation, giving conclusive results on an 

 ancient and fundamental problem, have grown branch studies by 

 Woodruff and his associates on a large number of diverse factors 

 affecting reproduction. This work has been done mainly on 

 Paramecium, as a contribution to the general effort to get the 

 genetic physiology of one type animal fully cleared up, but other 

 infusoria have likewise been dealt with. We may divide these 

 studies into (1) those on internal factors and (2) those on exter- 

 nal factors. 



1. Internal Factors. — In his first paper (1) as we saw, Wood- 

 ruff distinguished certain small changes in the reproductive rate, 

 which he called rhythms. The question comes up as to whether 

 these, like the changes resulting in death, may not be due to 

 something in the environmental condition, perhaps to fluctua- 

 tions in these conditions. This problem was attacked by Wood- 

 ruff and Baitsell (16). Their result was that practically con- 

 stant conditions of the environment tend to bring out the 

 rhythms more clearly, from which it is concluded that they are 

 due to causes w ithin the organism. A possible chance for doubt 

 of this conclusion arises from the question whether the pre- 

 cautions taken to keep the bacterial content of the cultures uni- 

 form were adequate ; certain work done in the Zoological Labora- 

 tory of the Johns Hopkins University indicates that they were 

 not— in which case the fluctuations in the reproductive rate 

 might be due to variations in the bacterial content of the 

 medium. 



Baitsell's study (13, 23) of the effects of conjugation between 

 closely related individuals in SI i/lon i/rh in /,</.<( nlala belongs here. 

 It was found that after such conjugations the animals do not 

 continue to reproduce. Baitsell summarizes as follows : 



The experiments show conclusively, it is believed, that conjugation is 



relation, in this form at least, to a particular period of the life cycle. 

 It is suggested that infertility of syzygies in these cultures is the 



history (23, page 74). 



With regard to the second suggested conclusion, doubt may be 

 raised, since it was not shown that under the conditions gametes 



