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THE AMERICAN NATURALIST 



[Vol. L 



state where the causes underlying regeneration are inac- 

 tive. This may be due to the exhaustion of specific sub- 

 stances which take part in the reaction of regeneration, or 

 it may be due to the chemical and physical changes accom- 

 panying cell division. 



We are led through these experiments, to further specu- 

 lations concerning the nature of cell division. Chemical 

 differentiation of the protoplasm continues even after the 

 stage is reached when regeneration is possible. This is 

 shown by the fact that formation of the cirri in Ur -onychia 

 precedes the process of division in normal cells, and by 

 the additional fact that regeneration of cirri occurs while 

 cell division does not occur in enucleate fragments cut 

 from old cells. I would interpret cell division as due to 

 cytolytic action set up by enzymes or other chemical 

 bodies produced as a result of interaction of nucleus and 

 cell body differentiated chemically by age. Cytolysis may 

 then occur more or less extensively throughout the entire 

 protoplasmic mass, but it is most active in the division 

 zone of the organism which is more highly differentiated 

 than other regions (see Calkins, 1911, and Peebles, 1912). 

 The membrane of the cell turns in at this cytolyzed divi- 

 sion zone and the constriction results in cell division. 



^ As a consequence of the activities accompanying cell 

 division the protoplasmic substratum is reduced from the 

 differentiated adult condition to the condition characteris- 

 tic of young cells, and the processes of growth and chem- 

 ical differentiation, division and de-differentiation, recur 

 m more or less rhythmical succession. 



Viewing the life cycle as a whole, there are two phases 

 which must be taken into account. These are, first, the 

 encystment phase, and second, the sexual or conjugation 

 phase, both widespread and almost universal in protozoan 

 life histories. Let us first consider the encystment phase. 



Encystment occurs ordinarily when the conditions in 

 the surrounding medium are adverse, such as desiccation, 

 lack of food, etc., such encysted forms emerging from the 

 cyst when suitable conditions are restored. In some cases 

 also, encystment occurs during the digestion of food. In 



