188 PROTOZOA 



information will be utilized in the following comparison of the 

 varieties of P. aurelia. 



1. Fertilization. Varietal differences with respect to fertiliza- 

 tion are of several sorts: (a) the kinds of fertilization processes 

 that occur; (b) the tempo of the processes associated with fer- 

 tilization; (c) the environmental conditions required for the 

 occurrence of conjugation; (d) the number of interbreeding mat- 

 ing types; and (e) the number of micronuclei and macronuclear 

 anlagen formed from products of the fertilization nucleus. In 

 previous sections, (d) and (e) have been set forth and will not 

 be further mentioned. 



(a) The kinds of fertilization processes. Basically there are 

 only two kinds of fertilization processes, conjugation and autog- 

 amy. Conjugation involves union of the animals in pairs. Each 

 mate undergoes meiosis and one of its reduced nuclei divides to 

 form a male and female gamete nucleus. The male nucleus of 

 each conjugant migrates into the mate with whose female nucleus 

 it unites to form a diploid fertilization nucleus. Autogamy in- 

 volves exactly the same nuclear processes as conjugation except 

 that cross fertilization is impossible because the animals are not 

 united in pairs. Instead, the fertilization nucleus is formed by 

 union of the male and female gamete nuclei of one and the same 

 individual. Both conjugation and autogamy can initiate a new 

 life cycle, but an appreciable period of immaturity never occurs 

 after autogamy. Conjugation occurs in all varieties. Autogamy 

 has been found in all varieties except varieties 13, 15, and 16. It 

 may yet be discovered in variety 13; 6 but it probably will not be 

 found to occur as a regular normal event in varieties 15 and 16. 



(b) The tempo of the processes associated with fertilization. 

 The time that conjugants remain united, i.e., approximately until 

 the time of the first division of the fertilization nucleus, varies 



nversely with temperature. At any one temperature, the time 

 varies with the variety. Thus, at 22°, conjugants remain united 

 for about 7 to 8 hours in variety 4, about 12 hours in varieties 13 

 and 15. Correspondingly, the time from separation of the con- 

 jugants until the first fission is about 15 hours in variety 4, 24 

 to 25 hours in varieties 13 and 15. In both cases the interval is 



