PROTOZOA 87 



plasm (Figs. 17 a and \lb) ; at the same time its macronucleus 

 breaks up and the fragments are distributed throughout the cyto- 

 plasm exactly as in the process of conjugation, while the micro- 

 nucleus divides and gives rise to a new macronucleus and to new 

 micronuclei. A phenomenon of reorganization, paralleling that 

 of conjugation, and termed endomixis, is thus an accompaniment 

 of encystment, and, upon emerging from the cyst, the young or- 

 ganism has a high potential of vitality similar to that of an 

 ex-conjugant. 



Some types of ciliates apparently never undergo encystment. 

 Thus, cysts of Paramecium are rarely seen, nevertheless it 

 undergoes the reorganization changes which characterize encyst- 

 ment in other ciliates, but without encysting. 



These three phenomena — cell division, endomixis, and con- 

 jugation — are unquestionably of the greatest importance for 

 the survival of protozoan protoplasm and represent a logical con- 

 sequence of the differentiation which accompanies continued me- 

 tabolism. Before discussing the line of argument which this 

 suggests let us glance at the metabolic activities which make up 

 the daily life of a protozoan. 



