186 PROTOZOA 



pendently determined for mating type. Hence, the unit of mating 

 type inheritance is the caryonide, the descendants that carry 

 products of division of one original macronucleus. Since usually 

 two new macronuclei arise in a fertilized animal and one passes 

 to each of the two products of the first fission, a clone includes 

 two caryonides that arise at the first fission after fertilization. 

 Because of the independent determination of the macronuclei, 

 the two caryonides from a fertilized animal often are different 

 in mating type. In group B this also happens sometimes, but so 

 rarely as to be inconspicuous. For a detailed account of the 

 similarities and differences between the group A and group B 

 systems, as well as of a number of exceptional phenomena that 

 are omitted here, the reader is referred to a review by Nanney 

 (1954). Further details and a more general interpretation will 

 also be given below in other connections. 



As a rule, the differences between the group A and group B 

 methods of determination and inheritance of mating type are so 

 striking that an unidentified strain could be assigned to one or 

 the other group on the basis of a study of half a dozen or so 

 pairs of conjugants and their vegetative descendants. It would be 

 required only to isolate the pairs, then to isolate the exconju- 

 gants, to cultivate separately the products of the first (better, the 

 first two) fissions, and finally to ascertain the mating types of the 

 various cultures. If the cultures from the two exconjugants of a 

 pair are usually of different mating types, the strain belongs to 

 a variety of group B. If the two caryonides from a single excon- 

 jugant often differ in mating type, or if the same mating type 

 frequently occurs among the descendants of both exconjugants 

 of a pair, the strain probably belongs to a variety of group A. 

 There is, however, one serious source of possible misinterpreta- 

 tion. In group B, if the two conjugants of a pair exchange much 

 cytoplasm, they may produce clones that arc alike in mating 

 type. This difficulty can be avoided by watching the conjugating 

 pairs as they conclude conjugation and eliminating those that 

 remain long united posteriorly after separating at the anterior 

 ends. Only those that remain long united posteriorly exchange 

 enough cytoplasm to affeel the inheritance of mating type. 



