SEXUALITY 697 



(Kimball, 1939a) of answering these questions directly. The two ani- 

 mals that come together for conjugation must be separated before they 

 become too tightly united, and the mating types of the two members of 

 such a split pair must be directly ascertained by placing each of them 

 separately in standard cultures of the different types, to discover with 

 which ones they will react sexually. If one reacts only with type I and 

 the other only with type II, they must be of different types; but if both 

 react with the same type, then they are alike in mating type. 



This problem has been most fully studied by Kimball ( 1939a, 1939b) . 

 In P. aurelia, he found that conjugation within a caryonide occurred un- 

 der two very different kinds of conditions. One kind is very common; it 

 occurs in caryonides genotypically of type I, when the last preceding 

 caryonide in the direct line of ancestry was of type II. Under these condi- 

 tions conjugation may occur in the caryonide during the first few days 

 of its existence. Kimball split some of these conjugant pairs, tested them 

 directly for mating type, and showed that in each pair one animal was of 

 type I, the other of type II. Thus both mating types can be present in a 

 single caryonide, and the mating is between the two types only. Kimball 

 now obtained clone cultures from the two members of such split pairs 

 and found in every pair that both cultures were of type I and showed no 

 further conjugation among their own members. Hence the type II ani- 

 mals originally present in the caryonide changed to type I. The early oc- 

 currence of type II was due to the type II character of the immediate 

 ancestors. This phenotypic or cytoplasmic "hang-over" fades out, as the 

 new genotype comes into action. Not all individuals accomplish this at 

 the same speed, so for a short time some are still type II while others 

 have completed the change to type I; at this moment conjugation may 

 occur. A little later all have changed to type I, and conjugation is no 

 longer possible. Similar "cytoplasmic lag" in the inheritance of other 

 characters in Paramecium had been reported by both De Garis (1935) 

 and by Sonneborn and Lynch (1934). 



The other type of conjugation within a caryonide is of much rarer 

 occurrence. In the race of P. aurelia examined by Kimball (1939b), less 

 than 3 percent of the caryonides showed it. In these, conjugation oc- 

 curred not only when the caryonide was young, but probably through- 

 out its whole history. Moreover, any individual in the caryonide gave rise 

 to progeny that conjugated with each other. Even the members of a split 



