FISSION RATE OF STYLONYCHIA PUSTULATA 453 



The paper of Jennings on the Effect of Conjugation in Para- 

 mecium ('13) hkewise deals to a certain extent with this matter. 

 In a wild population many strains differing in rate of fission 

 (under the same conditions) were found to occur. Furthermore, 

 it was demonstrated that even in a population derived by fission 

 from a single individual (that is, iri a 'pure strain'), conjugation 

 produced inherited differences in the fission rate, so that after 

 conjugation there were present strains showing constant differ- 

 ences in these respects. 



On the other hand, if no conjugation has occurred among the 

 progeny of a single individual, the fission rate was found to be 

 nearly or quite uniform. Jennings sums up as follows: 



It is found (1) that differences in rate of fission among those that 

 have not conjugated since they were derived from a single parent are 

 hot inherited (unless possibly certain differences of a minimal character 

 are to be excepted; differences of an order of magnitude far below those 

 with which we are dealing); (2) that conjugation among the members 

 of such a pure race does result in differentiations that are inherited 

 ('13, p. 366). 



The paper of Calkins and Gregory ('13) on the other hand 

 sets forth that there are in many cases differences in the fission 

 rate among the four sets of progeny resulting from the first two 

 divisions of an individual that has just conjugated. 



Other papers bearing less directly on this matter will be taken 

 up in the discussion of the results of the present work. 



The specific problem 



When a single infusorian divides, often one of its two progeny 

 again divides before the other does. In successive generations 

 this same thing may be repeated. Thus, as shown in figure 1, 

 one may have among the progeny of a single individual at a 

 given moment, animals that are the products of four and others 

 that are the products of two fissions. Hence there are differ- 

 ences of fission rate among the descendants of a single indi- 

 vidual — differences that afford the opportunity of selection with 

 a frequency that made it appear worth while to determine 

 whether these differences are inherited and whether slow lines 



