478 BIOLOGY OF THE PROTOZOA 



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 concei^^able that 

 environmental media may induce different protoplasmic reactions 

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

 experiments with Uruleptus, and that proper salts in the medium 

 at appropriate periods would enable the protoplasm to maintain 

 its youthful labile condition. Individnals 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 

 A^ariable diet, the medium being changed at intervals but in this 

 case without difference in his results. In this connection it is 

 possible that old protoplasm might be reorganized by increasing 

 the permeability and with proper interaction between protoplasm 

 and medium, restored to its original labile condition. 



Enriques (1903, 1905, 1916) using Stylonychia, Oxytricha and 

 Glaucoma, found that continued reproduction of these ciliates is 

 possi})le so long as one cares to follow^ it provided the "technic 

 is good and bacteria are not too numerous." The physiological 

 deterioration in isolation cultures he attributes to harmful products 

 of bacteria of the medium and maintains, although jjroof is not 

 given, that neither parthenogenesis nor conjugation is necessary for 

 continued life. His best evidence was obtained with Glaucoma 

 jjyriformis (1916) which he cultivated for eight months. During 

 this period 2700 generations were recorded, frequently as many as 

 13 per day and at no time did the organism divide less than 9 times 

 per day (!). It is quite possible as Jennings points out (1920) 

 that endomixis might have occurred without evidence of depression 

 and in the absence of cytological study, was overlooked; or it is 

 also possible that his eight months of observation with the high 

 vitality evidently possessed by this organism, covered only a small 

 l)art of the entire life history. 



With Didinium nasvtum divergent results have been obtained 

 by Calkins (1913) and Mast (1917). The former found that the 

 division-rate descends from an optimum immediately after encyst- 

 ment to a low miniminu just prior to the next encystment and that 

 a fairly uniform number of generations intervenes between encyst- 

 ment periods. The latter gives no data from which the division- 

 rate can be computed but finds a much longer interval between 

 certain encystment periods (conjugation may have occurred), 

 while other intervals agree with those found by Calkins. No 

 evidence was adduced by Mast to indicate cyclical changes in the 

 division-rate but indications of reduced vitality were evident from 

 the statement that "the stock became very weak toward the close— 

 and it is not known how much longer it would have survived." 

 (loc. cit. J). 353). 



