FERTILIZATION 



635 



Figure 152. Chilodonella uncinatus. Migration of the pronuclei across the protoplasmic 

 bridge. Each pronucleus contains two haploid chromosomes still attached by strands to 

 their sister halves. Also visible in each conjugant are old and new oral baskets and the 

 granular remnants of the old macronucleus. (After MacDougall, 1925.) 



STAGES F, G, AND H, THE EXCONJUGANTS 



The subject of reorganization is dealt with elsewhere in this volume, 

 but we may consider briefly some of the cytological aspects of the re- 

 organizing exconjugant. 



After fusion of the pronuclei, the fertilization nucleus divides one or 

 more times, and from the products of division the new micronuclear 

 and macronuclear elements are formed, while extra products disintegrate. 

 The number of divisions the synkaryon undergoes before differentia- 

 tion of the macronuclei and micronuclei in various ciliates is reviewed 

 by Kidder (1933b) in his work on Ancistruma. Kidder lists eight 

 species in "group A," in which the micronucleus and the macronucleus 

 are differentiated after the first amphinuclear division. To these we may 

 add Chilodonella cucullulus (Ivanic, 1933); C. chattoni, C. labiata, C. 

 caudata, C. faurii (MacDougall, 1936); and Nyctotherus cordiformis 

 (Wichterman, 1937). In other ciliates, differentiation occurs after the 

 second amphinuclear division. Kidder lists twenty-one species in this 

 "group B," which includes a majority of the best-known ciliates. To 

 this list may be added Balantidhm (Nelson, 1934), from the Chim- 

 panzee. 



In about half of these species, all four products remain functional, 



