372 University of California Puhlications in Zoology [Vol. 18 



organs are known to go through a disintegration and the whole body 

 to undergo a reorganization. By subjecting the dividing ciliates, flat- 

 worms, and segmenting eggs to potassium cyanide, Child proved that 

 at the time of division when the cells were in the dedifferentiated con- 

 dition a period of rejuvenescence followed, which was a time when 

 the rate of metabolism was high. This he claims was an indication 

 that the tissue was physiologically young and most susceptible to 

 external stimuli. 



This idea of rejuvenescence following the period of dedifferentia- 

 tion probably serves as a basis for explaining why regeneration in 

 Uronychia occurs more readily in dividing individuals. The cytoplasm 

 is young and so responds readily to the call to complete the organisms. 

 Likewise, perhaps, we can explain the nuclear changes occurring in 

 Euploies. Basing our assumption on the evidence brought forth by 

 Child for different forms, we assume that at the time of division the 

 cytoplasm of Euplotes has undergone certain dedifferentiating changes 

 and has assumed a physiological young condition with a greatly in- 

 creased metabolism. Such being the case it would be necessary for the 

 macronucleus, the important vegetative organelle of the animal, to 

 undergo corresponding changes of dedifferentiation and rejuvenescence 

 in order to properly control the rejuvenated cytoplasm. These changes 

 in the nucleus are undoubtedly of two kinds, physical and chemical. 

 The physical changes we are able to witness, while the only evidence 

 of the chemical changes is the difference in staining reactions, which 

 with the stains we have used tell us practically nothing of the nature 

 of this very important phase of the whole reorganization process. 



Formation of New^ Cirri 



A detailed description of the way in which new cirri form is 

 scarcely necessary, since Griffin (1910) gives a very accurate account 

 of the whole process as found in E. worcesferi, and the description as 

 given for that species will well serve for the process as it occurs in 

 E. patella: the only noticeable point of difference being that E. patella 

 has only nine ventral cirri anterior to the anal cirri, while the other 

 species has ten. 



In Euplotes patella, as in other species of Euplotes, the new cirri 

 arise in two groups and all of the old cirri degenerate. Each group 

 begins as five depressions in the pellicle, one group immediately 

 anterior to the anal cirri and the other group still farther forward. 



