T. M. SONNEBORN 251 



(2) in nearly all cases, these two clones are alike in mating type. 

 Yet, when he tried to formulate genotypes which would agree 

 with the observed results of crosses, he was totally unable to do 

 so. The chief difficulty was that synclones of all possible mating 

 types were producible from the cross of any two mating types, 

 indeed from any single type which underwent self-differentiation. 

 For, like other matings, selfings also could produce all possible 

 types of synclones. 



Sonneborn (1947, p. 340) pointed out that the inheritance of 

 mating type in P. bursaria conforms in principle to the group B 

 pattern in P. aurelia when there is massive exchange of cytoplasm 

 between mates. The only difference is that there are four or 

 eight alternative types in most varieties instead of just two. Son- 

 neborn (p. 335) further cited three evidences that cytoplasmic 

 exchange between mates does in fact occur regularly in P. bur- 

 saria. ( 1 ) Harrison and Fowler ( 1946 ) observed transfer of anti- 

 gens from one mate to the other during conjugation. (2) As men- 

 tioned earlier, the abnormalities that occur during conjugation 

 between varieties 2 and 4 appear to be due to interactions be- 

 tween the nucleus of each variety and the cytoplasm of the other, 

 and they occur before nuclei are exchanged ( Chen, 1946a ) . ( 3 ) 

 Jennings ( 1944b ) observed that conjugation between an old and 

 a young clone usually kills both mates, abnormalities appearing 

 during conjugation itself. The evidence subsequently obtained 

 by Sonneborn and Schneller (1955) that nuclear abnormalities 

 are induced by old cytoplasm reinforces my interpretation of the 

 dependence of death of the young mate in Jennings' crosses on 

 the action of cytoplasm received from the old mate. Since cyto- 

 plasmic exchange is relatively rare in P. aurelia, usually the old 

 mate dies and the young one lives. To these evidences may now 

 be added the fact that Chen's numerous figures of conjugation 

 stages indicate the regular occurrence of a wide region of free 

 cytoplasmic continuity between the mates. There can thus be 

 little doubt that massive exchange of cytoplasm between mates 

 is the rule in P. bursaria. 



This then provides the explanation for the uniformity of mating 

 type in a synclone. Nanney (1956b) has come to the same con- 



