88 



Reproduction and Life-Cycles 



synkaryon divides one or more times. Some or all of the resulting nuclei 

 may differentiate into macronuclei and micronuclei. Only one nuclear 

 division precedes differentiation in Nyctotherus cordifortnis (242), several 

 species of Chilodonella (167), and a few other ciliates (143). Differentia- 

 tion occurs after the second division in Paramecium aurelia (70), Eiiplotes 

 eurystomus (234), and about twenty other species (143). Differentiation 

 follows the third division in Bwsaria truncateUa (205), O pisthonecta 

 henneguyi (211), Parachaenia myae (154), Vorticella microstoma (88), 

 Paramecium bursaria (57), P. trichium (72), P. caiidatum, and a number 

 of other species (143). Differentiation after a fourth postzygotic division 

 has been reported in Kidderia mytili (143) and Parameciuvi multimicro- 

 nucleatum (159). Behavior of the nuclei in ciliates showing two or more 

 postzygotic divisions differs from species to species. All of the nuclei may 



Fig. 2. 21. Development of a new macronucleus following conjugation in 

 Nyctotherus cordiformis; A-D, xllSO; E, x765 (after Wichterman). 



remain functional, or some of them may degenerate. Variations may occur 

 also in individual species, as in P. caiidatum (71) and P. trichium (72). 



Development of the micronucleus usually involves a decrease in size, 

 whereas a differentiating macronucleus grows and often undergoes ex- 

 tensive changes in form as well as internal organization. The young 

 macronucleus of Nyctotherus cordiformis (Fig. 2. 21) soon becomes finely 

 granular and stains more intensely. Later, the granules give rise to threads 

 during growth of the nucleus and then, as differentiation nears comple- 

 tion, the threads are replaced by the granules characteristic of the mature 

 macronucleus. The early stages of differentiation are similar in Euplotes 

 eurystomus. After the threads are replaced by granules the developing 

 macronucleus elongates, extends posteriorly, and makes contact with a 

 remnant of the old macronucleus. Fusion results in a complete macro- 

 nucleus (234). 



Depending upon the species, postconjugant fissions may or may not be 



