256 BIOLOGY OF THE PROTOZOA 



tions of culture, and in such cases he believes that natural environ- 

 mental conditions have been supplied. He obtained some cases of 

 greater longevity in a few series of Spathidium, and although the 

 methods and the culture medium supplied did not differ in any way 

 from those used in the series that showed decline and death, he 

 concludes that somehow the conditions were more suitable, and 

 that when suitable the ciliate has the ability or potential for an 

 indefinitely continued existence without the necessity of conjugation 

 (fertilization) or of an equivalent process. 



Chatton (1921 ) shares this scepticism : " One may even conclude," 

 he says, "that the more the facts accumulate, especially those of 

 an experimental nature, the more nebulous does this conception of 

 a life cycle (in filiates) become" (loc. cit. p. 128). The "facts" 

 thus mentioned include the exceptional results with experimental 

 culture methods by Woodruff as above, by Baitsell, Dawson, 

 Enriques, Mast and others, these being the most prominent, in 

 connection with the Infusoria. It is quite possible, as M. Robertson 

 (1929) brings out, that conditions of the milieu are such that stimuli 

 from the environment which ordinarily call forth adaptive changes 

 in the organization are not developed. 



In a similar manner Dawson (1919) found that an amicronucleate 

 race of Oxytricha hymenostoma presents a typical cyclical curve 

 of vitality, and death follows a gradually decreasing vitality, if the 

 organisms are cultivated in isolation cultures. If maintained in 

 mass cultures they were found to live for a considerable period 

 longer than the isolated forms, and Dawson concludes that if a 

 suitable medium is provided an indefinite life is possible without 

 conjugation, endomixis or encystment. It is conceivable that 

 environmental media may induce different protoplasmic reactions 

 at different periods of the life cycle, as shown by Gregory's (1925) 

 experiments with Uroleptus, and that proper salts in the medium 

 at appropriate periods would enable the protoplasm to maintain its 

 youthful labile condition. Individuals might thus be "doctored" 

 at intervals with a resulting repression of cumulative differentiations 

 and a corresponding maintenance of youth. This was the under- 

 lying principle of Woodruff's cultivation of Paramecium aurelia on 

 a variable diet, the medium being changed at intervals but in this 

 case without difference in his results. Austin (1927) likewise, sub- 

 jecting Uroleptus mobilis to different media throughout entire cycles, 

 was unable to alter the usual history. It is possible that old pro- 

 toplasm might be reorganized by increasing the permeability and 

 with proper interaction between protoplasm and medium, restored 

 to its original labile condition. 



In other groups than the ciliates, exceptions to the type of life 

 history shown by Uroleptus are true of the few cases known. In 

 the animal flagellates for example there is no case of indubitable 



