CONDITIONS FOR CONJUGATION 371 



during which period the culture 39a had lain dormant. Both 

 cultures were examined daily during the two weeks preceding 

 the experiment, but showed no conjugants. The mean division- 

 rate in fifty-two lines from 39 A was found to be 11.25 ± .175 

 for a twelve-day period; the mean rate in forty-four lines from 

 39a was 15.70 i .193 for the same twelve-day period. The 

 effect of long-deferred division, i.e., dormancy, is, then, to render 

 the organisms capable of more rapid division when favorable 

 growth conditions are afforded. It follows from these results 

 that dormancy may influence conjugation through its indirect 

 effect on fission-rate, if it be true that rapid division induces 

 conjugation. This may serve to explain, also, why it is that con- 

 jugation occurs most readily in cultures of infusoria which have 

 but recently been brought into the laboratory and subjected to 

 conditions producing rapid growth. For obviously the conditions 

 in ponds and streams are not ordinarily such as to maintain these 

 animals in constant division, as we find in a fermenting hay cul- 

 ture, so that when brought into the laboratory they are in essen- 

 tially a dormant condition, and will multiply rapidly when the 

 conditions are right. On the other hand, the tendency for conju- 

 gation to become suppressed in previously conjugating strains, 

 after prolonged cultivation (p. 368), receives an explanation: 

 it is a phenomenon accompanying the gradual loss of the power 

 of division. 



A third line of evidence favoring the view that conjugation is 

 initiated by a period of unregulated division is obtained from the 

 records of fission-rate in strains which conjugate at regular 

 intervals. As pointed out by Hertwig ('89) and fully demon- 

 strated by Jennings ('13), the most apparent immediate effect, 

 produced by conjugation is a decrease in fission-rate. This 

 lowering tends to disappear completely in about two months 

 (P. caudatum). My observations on conjugating strains indicate 

 a similar condition which may be repeated after each recurrence 

 of the process; and this has led me to think that the periodicity 

 in conjugation may in some way be controlled by this tendency 

 for fission-rate to increase gradually to a high point. This is 

 the essential idea involved in Hertwig' s explanation of conjuga- 



