T. M. SONNEBORN 249 



and in P. caudatum. In the latter, stable types may arise in this 

 way. In the former, the differentiation can occur in variety 15 in 

 amicronncleate caryonides which are therefore obviously incapa- 

 ble of undergoing autogamy. Further, Erdmann's observations 

 were fragmentary and not well documented, and all could have 

 been stages of reorganizing exconjugants, the conjugation of 

 which had been overlooked. Finally, Chen's long and careful 

 observations on P. bursaria have never included a single instance 

 of autogamy in unpaired, i.e., non-mating, animals. It might be 

 possible now, if Lee's (1949) method of x-irradiation actually 

 induces self -differentiation, as he suggests, to put to a direct test 

 the question of whether autogamy precedes self-differentiation. 

 Previously, this was impossible because of its great rarity and 

 unpredictability. Meanwhile, until better evidence is forthcom- 

 ing, autogamy in single animals must be considered as not only 

 undemonstrated in P. bursaria, but as probably nonexistent. 



Thus, as matters now stand, P. bursaria conforms to the pattern 

 of outbreeders by lacking autogamy and by using in its place self- 

 differentiation and conjugation between the mating types thus 

 differentiated, to restore vigor to clones that have reached senility 

 without finding opportunities for outcrossing. 



4. Survival after inbreeding and outbreeding. The comments 

 made above about the relation of parental age to the frequency 

 of death after conjugation are correct, but the situation is greatly 

 complicated by the fact that other factors also influence the death 

 rate after conjugation. A most important factor is the relationship 

 of the parent clones themselves. In general, with one exception 

 to be noted later, outcrosses yield better survival than crosses be- 

 tween relatives. Jennings noted 5 to 14 or more times as much 

 death after conjugation between sibs (i.e., crosses between dif- 

 ferent synclones derived from the cross of the same two parental 

 clones) as after conjugation between clones derived from differ- 

 ent natural populations. Such comparisons were of course made 

 with clones of similar age. These relations obviously reinforce the 

 conclusion that P. bursaria is an outbreeder. They suggest that 

 the organisms are normally heterozygous for lethal and detrimen- 

 tal genes which come to expression through inbreeding and are 



