The Life-History of Hypotrtchous Infusoria. 627 



as measured by the rate of division. This cydical change is most 

 prominent in the Oxytricha A-culture. The periods of depres- 

 sion lead to death if the culture is subjected continuously to the 

 same environment. \ 



4. Minor fluctuations occur in the division-rate which I have 

 termed "rhythms" and which are to be clearly distinguished 

 from cycles. The rhythms are probably indicative of a rhythmical 

 change in the metabolism of the organism, though they are 

 influenced somewhat by almost imperceptible changes in the 

 environment. 



5. The results of the experiments seem to indicate that 

 "rhythms" and "cycles" should be defined as follows: 



A rhythm is a minor periodic rise and fall of the fission-rate, due 

 to some unknown factor in cell-metabolism, from which recovery 

 is autonomous. 



A cycle is a periodic rise and fall of the fission-rate, extending 

 over a varying number of rhythms, and ending in the extinction 

 of the race unless it is "rejuvenated" by conjugation or by changed 

 environment. 



6. Changes in the environment will revive the lagging func- 

 tions during the descending cycle, as is shown conclusivelv by the 

 sudden recuperation of Oxytricha A during July, 1902. There 

 is every reason to believe that this "rejuvenescence" was produced 

 by treatment with extract of beef. 



7. Seasonal and temperature changes have no apparent 

 influence on the cyclical fluctuations of vitality. Variation in 

 temperature, however, undoubtedly afi'ects somewhat the daily 

 rate of division, if not directly, at least through the food supply. 



8. The number of generations which constitute a cycle is not 

 at all constant; and there is no evidence to show that duration in 

 time is of any significance in the forms studied. 



9. Periods of extreme depression of vitality are manifested 

 on the physiological side chiefly by a greatly decreased division- 

 rate, and by the comparative frequency of pathological divisions. 

 Morphological changes are apparent chiefly in (i) an increased 

 vacuolization of the cytoplasm; (2) distortion and fragmentation 

 of the macronuclei; (3) numerical increase of the micronuclei; 

 and finally (4) in a reduction of the ciliary apparatus. 



10. Variation in the size of the infusorians during the life- 

 cycle is marked; the organisms being very small during periods 



