INHERITANCE 755 



conjugants originally of type II, 4 gave descendants of type II only, 3 

 of type I only, and 18 half of type I, half of type II. 



In summary, there were 47 ex-conjugants, which give 94 descendant 

 clones. Of these 94, 46 were of the same type as their parent's before 

 conjugation, 48 of different type. Thus it is clear that ex-conjugants 

 originally of a given type produce descendants of both mating types with 

 equal frequency. The reception in conjugation of a pronucleus from an 

 individual of different type changes the type as frequently as it leaves it 

 unchanged. There is no tendency for the descendants to be of the same 

 type as was the cytoplasmic body from which they are derived (certain 

 exceptions will be described later). 



Of the 47 ex-conjugants just considered, 28 gave two clones of dif- 

 ferent types, while 19 gave two clones of like type. There is no indica- 

 tion of a tendency for the two clones descended from a single ex-con- 

 jugant to be alike in type. 



Thus the first fission after conjugation separates the ex-con jugant into 

 two individuals which may be of different mating type, giving rise to 

 two clones of different types; or they may be of the same type. Segre- 

 gation of the diverse mating types (in cases in which it occurs) takes 

 place at the first fission after conjugation. 



What is it that decides the mating type of each clone? Sonneborn is 

 disposed to believe that the segregation is the result of the separation of 

 the two macronuclei, one macronucleus tending to produce type I, the 

 other type II. How the two became diverse (if this is the case) is not 

 known. There appears to be no evidence of a reduction division at this 

 point, such as might give rise to nuclei differing in chromosomes. It 

 appears equally difficult to suppose that the two nuclei of the same cell 

 are subjected to differing conditions, such as to cause one to be of type I, 

 the other of type II. 



Sonneborn (1938) has discovered, however, that the temperature 

 during conjugation affects the proportion of the types produced in a group 

 of ex-conjugants. In variety or group 1, higher temperatures cause the 

 appearance of a greater proportion of type II; in group 3, higher tempera- 

 tures favor the production of type VI (rather than type V) . But it is not 

 evident how the difference of type in the two products of fission of a 

 single individual could be induced in any such manner. The segregation 

 of types at the first fission after conjugation remains a riddle. 



