94 Reproduction and Life-Cycles 



meciiim bursaria (54). Fluid from cultures of several Russian strains (type 

 T) induces conjugation within individual mating types of varieties II, 

 III, IV and VI, although the effect is usually limited to a small percentage 

 of the ciliates in a culture. 



The recognition of mating types in certain ciliates has shown that con- 

 jugating pairs, in these species at least, are composed of physiologically 

 different organisms. However, the relation of mating types to the concept 

 of bisexuality in animals remains uncertain in Paramecium bursaria and 

 Euplotes patella. On the other hand, P. aurelia and P. caiiclatum might 

 possibly be interpreted as species composed of "bisexual" varieties which 

 interbreed with difficulty or not at all. 



Nuclear phenomena of uncertain significance 



Endomixis (250) was originally described in Paramecium aurelia 

 as a complete nuclear reorganization occurring in individual ciliates 

 (251). Macronuclear disintegration and two micronuclear divisions occur 

 without the usual third pregamic division of conjugation. Only two of 

 these eight micronuclear derivatives persist, so that the first fission leaves 

 each ciliate with one functional nucleus. Two nuclear divisions occur. 

 Two of the products then differentiate into macronuclei, while the others 

 divide to form four micronuclei. A second fission completes the reorgan- 

 ization. 



The significance of endomixis in the life-cycle is still unknown. Wood- 

 ruff believed that meiosis does not occur — although the second pregamic 

 division is reductional in conjugation of P. aurelia — and he suggested 

 that endomixis might be analogous to diploid parthenogenesis. The dis- 

 covery of autogamy in P. aurelia (70) and the accumulation of genetic 

 data have thrown doubt upon the occurrence of endomixis in P. aurelia. 



Hemixis involves unusual behavior of the macronucleus only. The 

 process has been observed in Paramecium aurelia, P. caudatum, and P. 

 multimicronucleatum. (70). In one type of hemixis there is a precocious 

 division of the macronucleus and the normal nuclear situation is restored 

 in the next fission. In another type, the macronucleus extrudes one or 

 more densely staining masses and then behaves normally in subsequent 

 fissions. A third type of hemixis combines the elimination of chromatic 

 material with precocious division of the macronucleus. 



THE PHYSIOLOGICAL LIFE-CYCLE 



The description of conjugation by O. F, Miiller in 1786 stimulated 

 much interest in the sexual activities of Protozoa. For many years, it was 

 believed that the "ovary" (macronucleus) of ciliates gave rise to "ova" 

 (products of macronuclear disintegration), while the "testis" (micro- 

 nucleus) produced "spermatozoa" (chromosomes). In conjugation, two 

 hermaphroditic ciliates were supposed to exchange spermatozoa. In cer- 



