Reproduction and Life-Cycles 



87 



ber participating in the pregamic divisions varies with the species. Only 

 one of the many micronuclei undergoes the first division in Dileptus 

 gigas (235), but two or more may do so in other species. Comparable dif- 

 ferences in nuclear behavior have also been reported for the second and 

 third divisions. Furthermore, variation may occur within a single species. 

 For instance, 2-5 (and possibly 1-5) products of the second pregamic di- 

 vision may complete the third division in Paramecium aurelia (70). In 



Fig. 2. 20. A. First pregamic division, early anaphase, Kidderia mytili; 

 xl875 (after Kidder). B. Late anaphase, second pregamic (reductional) divi- 

 sion, K. mytili; x2100 (after Kidder). C-G. Chilodonella uiiciimtiis: nuclei 

 just before the third pregamic division (C); late third division (D); fusion 

 of pronuclei (E); first division of the synkaryon (F, G); diagrammatic (after 

 MacDougall). 



any case, the nuclei which do not undergo a particular division in the 

 series soon degenerate. 



In typical conjugation the third pregamic division produces two or 

 more pronuclei. One of these, a migratory pronucleus, passes into the 

 opposite conjugant and fuses with a stationary pronucleus to form a 

 synkaryon (Fig. 2. 20, E). The actual exchange of pronuclei, which has 

 been questioned occasionally, is supported by recent cytological and 

 genetic data and has been observed in living specimens of Paramecium 

 bursaria (244). 



Fusion of the pronuclei is followed by a reorganization in which the 



