288 
H. S. JENNINGS 
INTERVAL BETWEEN CONJUGATIONS 
In a given culture of the races k or kh, it is usually not difficult 
to cause by proper treatment epidemics of conjugation to appear 
at intervals of two or three weeks. T have never observed a period 
when this could noc be done readily. The long intervals between 
some of the dates of observed conjugations, given above, are due 
merely to the fact that during those periods observation and ex- 
periment was not directed upon this matter. 
If we isolate in each case a single member of a pair, and under- 
take to determine how long it may be before a new conjugation 
may occur among its progeny, we find the problem a little more 
difficult. It is necessary to cultivate the animals for a time with 
care, or they may be lost. As shown by the diagrams of pedigrees 
given above, the interval may be short; periods of one month; of 
two weeks, are shown in certain cases. With proper care, it is 
probable that the period could be brought regularly to little more 
than two weeks. In one case there is a period of hut five days be- 
tween the conjugation of the parent and that of the progeny 
(fig. 3). The ex-conjugant had divided but four times, into 16 
progeny, when these progeny conjugated anew. 
CONDITIONS UNDER WHICH CONJUGATION OCCURS 
The great readiness to conjugate in the strain k gave an excel- 
lent opportunity for observation and experiment upon this point. 
In general, conjugation occurred in k under the following con- 
ditions: To a hay culture in which the infusion was getting ^'old,'' 
and in which the animals were rather thin and were not multi- 
plying rapidly, a handful of fresh hay was added. The Paramecia 
at once began to get plump and to multiply. This is due to the 
fact that soluble substances from the hay diffuse into the water, 
increasing the growth of bacteria, on which the animals feed and 
apparently also directly increasing the growth of the Paramecia. 
Three or four days after the hay was added, conjugation began, 
the epidemic often lasting several days. At the time of conjuga- 
tion it was noticeable that the animals had become thinner again, 
