510 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



based on specimens that were evidently adults; possible confusion 

 due to growth differences could be avoided. 



On March 31 I killed and measured all but the largest and 

 smallest pairs of conjugants; the length was found to vary from 124 

 to 148 microns. The smallest and largest pairs were reserved for 

 propagating; the former, of course, measured not more than 124 

 microns, the latter not less than 148 microns. These were allowed to 

 multiply separately, but under the same conditions, till April 10. 



On April 10 I measured a random sample of 100 specimens of 

 the progeny of each of these pairs. The results are as follows: 

 Larger pair, 151.920X43-840 microns. 

 Smaller pair, 158.760 X 38.120 microns. 



Thus, the difference in size, whatever its cause, does not corre- 

 spond to the difference between the ancestors ; selection for size has 

 had no evident effect. 



Another experiment on the progeny of Nf2 consisted in com- 

 paring the descendants of a single small conjugant with those of 

 several large non-con jugants. Details of this and similar experi- 

 ments will be reserved for our paper on the relation of conjugation 

 to variation and heredity. But since it has a certain bearing on our 

 present problem, the results may be given here. 



At the same time with the cultures last described (on March 31), 

 I isolated ten of the largest non-con jugant progeny of the same 

 individual Nf2. A sample of thirty-four of these had given a mean 

 length of 147.412 microns, so that this may be taken as the mean 

 length of these ten specimens. With the progeny of these was com- 

 pared the progeny of the smaller pair mentioned in the preceding 

 experiment. As we have seen, this pair measured not more than 

 124 microns in length. The greatest pains were taken to cultivate 

 the two sets under identical conditions. On April 20 I killed a 

 sample of 108 of each. The mean measurements were as follows: 

 Progeny of small pair (124 microns) — 160.852 X 42.036 microns. 

 Progeny of ten large (147 microns) — 156.482X43-815 microns. 



Thus, again, there is no correspondence between the differences 

 in size of the parents and those of the progeny. The determining 

 factor in the size is the fact that both sets belong to the same pure 



