6o2 Lorande Loss Woodruff. 



was wrong and that with a constant medium all rhythms could be 

 removed. To test it still further the same results were plotted 

 for five-day periods. This brought the rhythms to view again 

 {cf. Diagram VI) in such beautiful regularity that it seems to me 

 to show beyond doubt that the rhythmical element of the division- 

 rate cannot be caused entirely by temperature changes or by 

 imperceptible fluctuations in the food supply, but that it is due, in 

 the last analysis, to factors of a more complex character. Varia- 

 tion in the rhythm of division is well known in the development of 

 the metazoon egg, and it has yet to be satisfactorily explained. 

 Towle ('04) in a recent paper on the effects of stimuli on Para- 

 moecium is led to make this interesting statement: "There may 

 even prove to be rhythmical changes in sensitiveness like those 

 described by Lyon ('02; '04) for cleaving eggs, and Scott ('03) for 

 unfertilized eggs. Something of this nature is indicated by the 

 fact that Paramoecia from the same culture vary in sensitiveness 

 from day to day." In my work on the eflPects of chemicals on 

 Infusoria I have found that individuals react differently at various 

 times to a given stimulus [cf. p. 616 et seq.) and I believe we have 

 the clue to these "changes in sensitiveness" manifested in the 

 rhythms of the fission-rate. 



A point of some interest in regard to the rhythms in the Oxy- 

 tricha A-culture is the fact that the slowest fission-rate of each 

 rhythm in the descending cycle is less than that of the slowest 

 rate of the preceding rhythm. In the ascending cycle also, the 

 slowest rate in each rhythm is greater than the slowest rate of the 

 preceding rhythm. 



So far we have not considered the long trend of the division- 

 rate, which I regard as the cycle as I believe that this is directly 

 comparable with the cycle of Calkins's Paramcecium cultures. 

 The cycle obviously extends over more generations in Oxytricha 

 than in Paramoecium though in both cases it is a variable number 

 both in the same culture and in different cultures of the same 

 species. Maupas's rather definite limits to the life-cycle are not 

 substantiated by this work as will be readily seen by comparing 

 the various culture-curves. It must be borne in mind, however, 

 that neither Maupas's chief cultures nor my own were started 

 with ex-conjugants, and therefore the number of generations does 

 not afford a just basis of comparison, since they indicate merely 

 the number of bipartitions since the culture began and in no sense 



