MAIlNCi lYPl 1)1 n RAIINAIION 271 



rhc result of rcorgani/ation in aiiiicronuclcatc lines. These observa- 

 tions sliow tliat new mating tvpcs arise whenever new macronuclei 

 arc fornicd from micronuclei and that new mating types do not 

 arise at nuclear reorganization when new macronuclei fail to form. 

 .Moreover, it is seen that mating type is perpetuated through any 

 piece of the old macronucleus. 



In some stocks mating types fail to segregate at the first post- 

 zvgotic division, but segregate at the second or third division. 

 In these stocks multiple macronuclear anlagen have been found 

 and mating type distribution in these, as well as in "normal" stocks, 

 is correlated with the distribution of independently developing mac- 

 ronuclear anlagen (Sonneborn, 1938, 1939). In rare cases in some 

 stocks mating type segregation continues and is clearly not due to 

 the segregation of diverse whole macronuclei (Kimball, 1939). Car- 

 yonides showing this kind of mating type segregation contain both 

 mating types and conjugation occurs regularly within the caryonides. 

 These are called, therefore, "selfing caryonides." A more detailed 

 consideration of these selfers will be presented later. Except for the 

 selfing caryonides, the facts of mating type inheritance in group A 

 clearly require the conclusion that macronuclear differences deter- 

 mine the two mating types. 



The relative frequencies with which the different mating types, 

 and hence the diverse macronuclei, arise after fertilization varies in 

 dependence upon the temperature prevailing at this time (Sonneborn, 

 1939, 1942). The relative frequency of the even mating type in- 

 creases with the temperature over the range of 12° to 32° C. Al- 

 though this implies some effect of temperature on the developing 

 macronuclei, the effect could be either direct or indirect, through 

 the mediation of some cytoplasmic constituent. 



Regardless of the temperature within this range, and conse- 

 quently regardless of the relative frequencies of the two mating 

 types, the two independently developing macronuclei in a single 

 reorganizing cell are independently determined as to which mating 

 type they \\'ill control. This is shown by the fact that the relative 

 frequencies of the various possible combinations for the tw^o macro- 

 nuclei agree with the calculations based on simple probability con- 

 siderations. 



An anomalous result was obtained when animals of variety 3 

 underwent both conjugation and subsequent reorganization below 



