306 SEX IN MICROORGANISMS 



1947), animals activated by "killer" fluids (Chen, 1945), and natu- 

 rally autogamous animals (Diller, 1936). Paroral cone formation 

 should be distinguished from the process of cone fusion, since the 

 presence of cones does not automatically insure fusion. Actual fusion 

 occurs only in conjugation and possibly certain cases of "pseudo self- 

 ing" (Hiwatashi, 1951a). The presence or absence of cones has not 

 been reported specifically in abnormally conjugating material. How- 

 ever, cones presumably form in conjugating amicronucleate animals 

 since pronuclei pass to these from normal mates (Chen, 1940c). A 

 similar situation obtains in "abbreviated" conjugation (Diller, 1949), 

 where micronuclei are exchanged at precocious stages of meiosis. It 

 is clear from these cases that paroral cone formation does not depend 

 upon the presence of a micronucleus or particular meiotic stage. 



Macrojjuclear Breakdown. The macronucleus breaks down in 

 a characteristic way (see Hertwig, 1889; Maupas, 1889; Metz, 1947; 

 Sonneborn, 1947, for figures) in conjugation, "pseudo selfing" (Metz, 

 1947), natural autogamy (Hertwig, 1914; Diller, 1936), and in 

 "killer" fluid activation (Freer, 1948). Presumably macronuclear 

 breakdown follows the normal pattern in conjugating amicronucleate 

 animals (Sonneborn, 1938b; Chen, 1940c) and therefore is not 

 dependent upon a micronucleus. Macronuclear breakdown followed 

 by some degree of macronuclear regeneration apparently can occur 

 without other obvious manifestations of activation (Diller, 1936). 

 However, such hemixis does not follow the normal breakdown pat- 

 tern, according to Diller's account, and therefore its relation, if any, 

 to activation is uncertain. 



Meiosis. None of the normal non-meiotic events of activation 

 depends upon the micronucleus, since all these apparently occur in 

 conjugating amicronucleate animals. The initiation of meiosis may 

 be sequential to holdfast substance formation and loss of mating reac- 

 tivity for the reasons given above, but its serial relation to other events 

 has not been determined. In any event, the meiotic process itself is 

 subject to many independent variations. It may consist of nothing 

 more than a swelling of the micronucleus in abnormal stocks (Chen, 

 1946d), or it may progress in normal or abnormal fashion through 

 one or both divisions in intervarietal crosses between normal stocks 

 of P. bursaria. Chen (1946c) suggests that this results from mixture 

 of incompatible cytoplasms. In "abbreviated" conjugation (P. caiida- 

 tiim) the meiotic process may be out of phase with other events 



