SEX IN PROTOZOA 225 



niicroniiclciir dixision, segregates these into two daughter cells, each 

 with the normal equipment of one macronucleus and two micro- 

 nuclei (12). The similarity between this process and conjugation is 

 very close. 



Sonneborn and his associates made extensive studies of the con- 

 ditions under which "endomixis" took place in Faramcciiivi aureVui. 

 x\fter his discovery of mating tvpes in this species (1937), Sonneborn 

 made genetic studies of the consequences of this type of nuclear re- 

 organization and found that in each case autogamy took place instead 

 of "endomixis" (Sonneborn, 1947). It remains to be seen how many 

 other cases of so-called "endomixis" may turn out to be autogamy or 

 parthenogenesis. 



Other Forvis of Nuclear Reorganization 



In addition to autogamy, Diller (1936) also described a series of 

 nuclear reorganizations of the macronucleus which did not involve 

 replacements from micronuclear sources. These reorganizations were 

 called hemixis and could take place according to several patterns. 

 Usually a partial or complete breakdown of the macronucleus occurs. 

 Larger fragments, when segregated into daughter cells by cell divi- 

 sions accompanied by micronuclear divisions, could develop into 

 functional macronuclei. When the entire macronucleus fragments 

 into balls, degeneration followed. 



A slightly different form of nuclear reorganization was described 

 by Sonneborn (1947) for F. anrelia. When animals undergoing con- 

 jugation or autogamy were kept at a temperature of 38 °C after 

 fertilization and during subsequent development, the growth of the 

 macronuclear anlagen was arrested, not to be resumed until several 

 cell divisions had ensued (Fig. AF). Because the micronuclei were 

 less affected, cell division could continue. During these divisions some 

 animals were separated which lacked any macronuclear anlagen (3). 

 In these, one or more fragments of the old macronucleus began to 

 grow, and, as they became segregated by subsequent divisions, they 

 eventually developed into full-grown macronuclei (4 to 7). Sonne- 

 born called this process macronuclear regeneration. 



division, each with one macronucleus and two micronuclei. 13, normal specimen of 

 P. caitdatJiui; 14, two micronuclei formed, macronucleus breaking up; 15, 16, three 

 and four micronuclei, fragments of macronucleus; 17, three micronuclear products 

 degenerating; 18, 19, two micronuclear divisions; 20, growth of two anlagen; 21, cell 

 division segregating one macronucleus and one micronucleus into each daughter; 

 22, normal animal. 



