246 PROTOZOA 



among the varieties, but that possibility needs to be kept in mind. 

 Particularly, one would like to know more about variety 3, which 

 is rarer and less widely distributed; and about variety 4, which 

 has only two mating types. A priori, I would expect that these, 

 if any, varieties might be less committed to outbreeding. The 

 main features of life and the breeding system in variety 1 follow. 



1. Immaturity. Conjugation is invariably followed by an im- 

 mature period. It never lasts less than twelve days and usually 

 lasts three to five months when the animals are well nourished. 

 If the organisms are placed for a time under poor nutritive condi- 

 tions during immaturity, this may disproportionately delay the 

 onset of maturity. For example, 18 days of such conditions, with 

 good nutritive conditions during the rest of the time, resulted 

 in a total immature period of 10 to 14 months, and some clones 

 failed to mature during a period of years. Translating these labo- 

 ratory findings into what might be expected in nature, immatu- 

 rity probably lasts at least a year, for under natural conditions 

 poor nutritive conditions are hardly avoidable for long and this, 

 combined with the lower fission rate usually to be expected, 

 would extend immaturity at least to the 10-to- 14-month range. 

 This feature of its life is alone sufficient to mark P. bursaria as an 

 outbreeder for the features that could balance it are lacking, as 

 will be shown. 



2. Maturity and mating types. The capacity to mate develops 

 gradually and reaches a peak at which it remains for very long 

 periods. In the next paragraph I shall define what brings maturity 

 to an end. Under laboratory conditions, maturity last about 3 

 years. In nature, it probably lasts much longer because of the 

 lower reproductive rate. As was mentioned in connection with 

 the outbreeding varieties .15 and 16 of P. aurelia, a very long 

 period of maturity is an adaptation to increase the probability 

 of finding a suitable mate. Further, like variety 16 of P. aurelia, 

 the varieties of P. bursaria (except variety 4) have multiple 

 mating types which serve to increase the probability of mating 

 when a mature stranger is encountered. Finally, unlike any va- 

 riety of P. aurelia, in about 97.4% of the cases all four caryonides 

 of a synclone are alike in mating type (Jennings, 1942). More- 



