694 SEXUALITY 



and conjugate readily when mixed with the proper type from another 

 stock. Mating types appear to be of universal occurrence in P. aurelia. 



Although not more than two mating types occur in any one stock, 

 more than two must exist in the species, for both mating types in some 

 stocks fail to conjugate with either of the types in certain other stocks. 

 Altogether, six different mating types have been found (Table 12). 

 One group of stocks contains types I and II; a second group contains 

 types III and IV; a third group, types V and VI. Conjugation takes place 

 only between the two types in the same group, never between types in 

 different groups. This sexual isolation of the three groups of stocks 

 makes them distinct genetical species or varieties; but they appear to be 

 morphologically alike, all conforming to the description of the taxo- 

 nomic species P. anrel'ia. However, they are physiologically diverse in a 

 number of ways. 



Each mating type is uniquely defined by the type with which it mates. 

 The mating type of a culture can be ascertained by mixing some of its 

 animals with standard cultures of each of the six mating types. With one, 

 and only one, of these it will conjugate. Its mating type is the other one 

 in the variety with which it mates. For example, if it mates with type V, 

 it belongs to variety 3 and is of mating type VI. 



In P. hursaria. Jennings (1938a, 1938b; 1939a, 1939b) reports some- 

 what different mating-type relations. Each stock of this species shows as 

 a rule only one mating type. As nuclear reorganization is extremely rare, 

 a stock is practically equivalent to a caryonide of P. aurelia. The mating 

 types fall into three different groups, or genetical species (a fact first 

 found in P. hursaria), with no conjugation between types in different 

 groups (Table 13). In group I occur the four mating types A to D; in 

 group II, the eight types E to M; in group III, the the four types N to 

 Q. In each group each mating type conjugates with all the other types in 

 that group. This system of multiple interbreeding types is in marked 

 contrast to the system of paired types in P. aurelia. To discover the group 

 to which a new stock belongs, it must be mixed with at least two types 

 from each of the three groups. It will conjugate with one or both of the 

 types from one group, not with any of the others. It belongs to the group 

 with which it conjugates. To discover its mating type, it must now be 

 mixed with all the types of this group until one is found with which it 

 will not conjugate. It is then of the same type as this one. For example, 



