T. M. SONNEBORN 243 



considerable period, abnormality suddenly develops. The chro- 

 mosomes clump, meiosis does not proceed beyond the first divi- 

 sion, and the nuclei acquire abnormal shapes and behavior. These 

 abnormalities are not consequences of an intervarietal hybrid 

 genotype, for the nuclear processes do not reach the stage of 

 fertilization. As Chen pointed out, death is due to an interaction 

 between the conjugants, which is dependent upon contact and 

 possibly upon migration of substances from one mate to the 

 other. The situation has features in common with the mutual 

 nucleocytoplasmic incompatibility reported by Melvin (1949) 

 and Levine ( 1953 ) for the cross between varieties 4 and 8 of P. 

 aurelia. Melvin reported that when cytoplasmic exchange was 

 induced (by exposure to antiserum), the pairs died before sepa- 

 rating. The isolation of varieties 2 and 4 of P. bursaria is also 

 probably to be understood as due to mutual nucleocytoplasmic 

 incompatibility. 



A viable Fl is thus never achieved in crosses between the 

 known varieties of P. bursaria. On the other hand, as in P. aurelia, 

 different strains of the same variety can interbreed freely and 

 yield (under proper conditions, see below) viable, normal prog- 

 eny. Hence, the varieties of P. bursaria, like those of P. aurelia, 

 conform to the modern biological concept of species. In our 

 terminology, each variety is a syngen. 



Varietal Differences. As with P. aurelia and P. caudatum, the 

 question now arises as to whether the varieties of P. bursaria 

 could be identified without recourse to standard living cultures 

 of the various mating types and if so, whether such identification 

 is sufficiently practicable to warrant assigning species names to 

 the varieties. In order to answer this question, the varietal differ- 

 ences will be summarized. 



1. Geographical distribution. Variety 1 occurs in China and 

 the United States ( Chen, 1956 ) ; varieties 2 and 3 in the United 

 States; varieties 4, 5, and 6 in Europe. Barring new extensions of 

 range and the discovery of additional varieties in the regions 

 explored, the problem of identification would be much simpler 

 in P. bursaria than in the other organisms thus far considered, for 

 not more than three varieties are known from any continent. If 



