2 18 PROTOZOA 



would lead to the conclusion that P. bursaria "needs" a long re- 

 productive period because sexual reproduction rarely occurs. 



Senility in P. bursaria may be marked by another feature, 

 which also has its analogy in the other species that have been 

 discussed. This is the phenomenon, mentioned earlier, which 

 Jennings (1941) discovered and called "sell-differentiation": a 

 clone previously pure for one mating type comes to contain two 

 mating types which can be isolated and shown to reproduce true 

 to type. Self-differentiation is a very rare event, occurring once in 

 about 2000 culture-days. It was not assigned to a definite stage in 

 the life cycle by Jennings; the available data are actually not 

 sufficient to make such an assignment with assurance. The uncer- 

 tainties are due to two facts. First, many of the observed cases 

 occurred in clones begun with a wild individual of unknown age. 

 Second, the clones of known age, in which self-differentiation 

 occurred, are heterogeneous with respect to the kind of matings 

 that produced them and there may be life cycle differences asso- 

 ciated with this. For example, clones produced by conjugation 

 between two mating types that arose by self-differentiation 

 within a single clone may themselves reach earlier than usual the 

 stage at which self-differentiation can occur. There is some indi- 

 cation of this in the data of Jennings, but the matter has not been 

 systematically explored. Tentatively, and largely by analogy with 

 other species, self-differentiation in P. bursaria may be considered 

 as a feature of senility which assures rejuvenescence for clones 

 that have failed to find suitable mates, by permitting conjugation 

 to occur within the clone. 



Jennings (1941) assumed that self-differentiation was probably 

 a consequence of prior autogamy. There were two reasons for 

 this assumption. First, the differentiation of stable mating types 

 within a single line of descent is due to autogamy in /'. aurclia. 

 Second, Erdmann ( 1927) briefly noted the occurrence of nuclear 

 reorganization within a clone of P. bursaria, and her observations 

 have been later interpreted as representing autogamy. However, 

 neither ol these reasons now seems cogent to me. Self-differentia- 

 tion oi mating types within a caryonide in the absence ol autog- 

 amy has been demonstrated in varieties 15 and 10' of P. aurclia 



