294 
H. S. JENNINGS 
before, while the last five were races that had been cultivated 
one to three years in the laboratory. In this infusion the ani- 
mals (each race of course in a separate vessel) multiplied rapidly. 
The infusion was changed frequently, all the cultures being treated 
in precisely the same way. On June 14, after all had multi- 
plied extensively, the fluid was diluted to one-third its usual 
strength, and was not changed again. After a few days all began 
to become thinner and the division rate declined. On June 19 
conjugation set in in kh, hut in no other strain. The cultures were 
kept until the animals finally died from starvation. 
This experiment brings out clearly the di^erence between the 
strain k and the other strains ; cultivated under precisely the same 
conditions, the individuals belonging to kh conjugate, while the 
rest do not. Yet kh had undergone epidemics of conjugation more 
recently, before this experiment, than any of the other races 
involved. 
It is clear therefore that different races of Paramecium differ 
extremely in their readiness to conjugate when favorable external 
conditions are supplied. Are the differences correlated with other 
noticeable differences in the races.'* As fig. 1 shows, some of the 
races are much larger than the others. Do the larger races, for 
example, conjugate less readily than the small ones: Or is the 
reverse the case? Examination shows that there is no constant 
relation between relative size and relative readiness to conjugate. 
The largest race, L2, conjugates rarely; the next largest, Z>, 
though long observed, has never been seen to conjugate. The 
smallest races, c and i, likewise conjugate only very rarely. The 
race k, which conjugates with such extraordinary frequency, is of 
intermediate size, though belonging with the smaller rather than 
with the larger group. The other two races in which conjugation 
was not rare, — g and C2 — were likewise races of intermediate 
size, differing Uttle from k in this respect. The three races, k, 
g and C2, that conjugate most readily, have two micronuclei. 
But c and i, which conjugate but rarely, have likewise two micro- 
nuclei, while L2 and D, also conjugating rarely, have but one.» 
' For determination of the number of micronuclei in the different races, I am 
indebted to Dr. George T. Hargitt. A joint paper by Dr. Hargitt and the present 
author, on the characteristics of the diverse races, will appear shortly. 
