LIFE HISTORY OF PRORODON GRISEUS. 315 



P. griseus differs from most Holotrichs, in the first stages of 

 conjugation, in that each conjugant contracts, becomes sperical, 

 and rotates vigorously. The sides in contact are somewhat 

 flattened, with their sub-terminal oral apertures facing each other. 

 They remain in this spherical shape until a point of cytoplasmic 

 union is complete (Fig. 70). Then they elongate (Fig. 3), and 

 move freely through the culture until conjugation is complete. 

 A greater number of the paired conjugants as a rule remain 

 contracted, rotate on their long axes and secrete a temporary cyst 

 in which they remain for the greater part or the entire period in 

 the completion of conjugation (Fig. 26). 



In this connection it may be of interest to note that no conju- 

 gating individuals were found out of doors or under wild con- 

 ditions. Although many of the pools were found in unfavorable 

 conditions due to evaporation. In fact very few free-swimming 

 forms were found at any one time when collections were made. 

 Most of the individuals were encysted. But when placed under 

 laboratory conditions, within a few hours the greater number of 

 individuals escaped from the cysts, and became quite active. 

 This was followed immediately by a regular epidemic of conju- 

 gation. Very few single individuals could be found in the 

 cultures. Again within three to five hours very few conjugating 

 individuals were present. The exconjugants during their free- 

 swimming period fed quite actively and reproduced by binary 

 fission. In about five hours encystment again occurred. Thus 

 beginning a new cycle. A few active forms however, can be found 

 in the cultures at all times. 



It is maintained by Calkins and others that encystment 

 (especially where cellular reorganization occurs), and conjugation 

 are both conducive to renewed vitality. While by Mast and 

 others it is held that neither encystment nor conjugation have any 

 appreciable effect in producing new vigor. 



In P. griseus however, we have conjugation immediately 

 following encystment or even the conjugants may encyst during 

 their nuclear exchange and reorganization. Here we might 

 conclude that the exconjugants would receive unusual renewed 

 vigor and increase the fission rate. This however, is not what we 

 find in P. griseus. It is true that an epidemic of cell division 



