Races of Paramecium. • 533 



jugated even when a period of conjugation occurred. The races 

 d and g, of the same size as k, likewise resemble k in conjugating 

 readily, though apparently less readily than k. 



In the caudatum group conjugation is throughout not readily 

 induced in animals living in the laboratory. On bringing material 

 from the open, the specimens of caudatum are often found conju- 

 gating in great epidemics, a short time after being placed in labora- 

 tory conditions. But later, conjugation becomes very rare. The 

 race D (three years and two months in the laboratory) has never 

 been seen to conjugate, though it has been observed more carefully 

 than any other race, and it has repeatedly been subjected to the 

 same conditions that induce conjugation so readily in k. The race 

 Li has shown two epidemics of conjugation, in the two years and 

 four months in the laboratory. It does not conjugate when sub- 

 jected experimentally to the conditions that induce conjugation 

 in k. But one other epidemic of conjugation has been observed 

 in a pure race of the caudatum group, when grown in the labora- 

 tory. 



The readiness to conjugate shows no uniform relation to the 

 relative sizes of the races. Those which conjugate most fre- 

 quently are the larger races of the aurelia group {k, C2, g). The 

 large races of the caudatum group (L2, D, etc.) and the smaller 

 ones of the aurelia group (c, i) cannot easily be induced to conju- 

 gate. 



4. Differences in rate of fission among the different races 



From March 5 to April 11, 1910 (thirty-seven days), an exten- 

 sive experiment was carried on for determining the rates of fission 

 in the different races. The conditions were kept as nearly as 

 possible absolutely uniform for the representatives of all the races, 

 and the exact number of fissions each day was recorded. 



The details of experimental procedure were as follows: Two 

 or more representatives of each race were isolated (Jn a concave 

 slide, a single individual in each concavity. Each was placed in 

 three drops of ' 'standard" hay infusion, made as follows: One 

 gram of pure Timothy hay (Phleum pratense) was boiled for ten 

 minutes in 50 cc. of water. To this was added 50 cc. of tap water, 



JouKNAi. OF Morphology, Vol. 21, No. 4. 



