The Life-Htstory of Hypotrichous Infusoria. 605 



of generations attained before "the initial potential of vitality" 

 is exhausted. Calkins's discovery of w^hat he calls "incipient 

 fertilization" in Paramoecium — that is, of two ex-conjugants 

 which continue to live "one is invariably far more vigorous than 

 the other" — would seem to bear out this point and to show that 

 the number of generations or the period over which a cycle extends 

 is not a point of great moment. 



Taken as a whole my cultures show conclusively that the three 

 species of hypotrichous ciliates studied are subject to periods of 

 greater and less dividing activity, and since the fission-rate is 

 probably a fair criterion of the metabolic activity of the protozoan 

 cell, that ciliates pass through alternating periods of greater and 

 less general vitality. This is also the general conclusion reached 

 by Engelmann, Biitschli, Maupas, Joukowsky, Simpson, and 

 Calkins, and from the range of species investigated it can probably 

 be accepted as of quite general occurrence among the Infusoria.^ 



2. Artificial Rejuvenescence. 



Calkins ('02, i) showed conclusively that Paramcecium cultures 

 when becoming extinct can be revived by the application of various 



'Peters ('04) working on Stentor, states that "neither direct observation nor the experiments made, 

 furnish evidence of any inherent periodicity of division. The present experiments show that, except 

 when some special modification of the medium exists (e. g., presence of potassium chlorid in excess), 

 multiplication runs, in the main, parallel to metabolism." Peters's experiments were not planned directly 

 to investigate this point and I fail to see, from his description of the methods employed, how cyclical 

 variation in the fission-rate, unless very pronounced, would be apparent. That "multiplication runs, 

 in the main, parallel to metabolism" is, I take it, not open to question, and is in no way opposed to 

 periodic fluctuations of the fission-rate. Peters says further, in regard to the culture medium employed 

 in determining periodicity of division, that "such promiscuous mixtures as hay infusion of unknovm 

 composition will not sufBce. Since frequent chemical analyses are impracticable, it will be necessary to 

 construct by trial artificial media of known composition." Undoubtedly hay infusion is not an ideal 

 culture-liquid, but when the hay or grass is carefully selected and thoroughly washed and otherwise 

 treated uniformly, and when this is prepared fresh each day and employed as soon as it is has reached 

 the room-temperature, there is little chance for fermentation, and I believe that about as near a perfect 

 medium is obtained as is practicable. Undoubtedly the ideal culture-liquid would be one artificially 

 combined so that its salt content, etc., is accurately known; but as Peters himself says, ". . . a food 

 supply must be added to the salt solution, and this requirement has proved to be a difficulty. For the 

 addition of any food that has been found available utterly changes the salt content both qualitatively 

 and in its proportions." To supply this demand Peters added to the artificial medium which he con- 

 cocted, "some dry leaves or dead reeds, or both. . . . The final step is to 'seed' this culture with a 

 mixture of all sorts of Infusoria, and other living material from thriving cultures." This done, I do 

 not see how a hay-infusion could be a more promiscuous mixture. 



