LIFE HISTORY OF PRORODON GRISEUS. 317 



temperature, scarcity of food and evaporation. The ciliate if 

 disturbed may remain in the temporary cyst less than an hour, 

 and immediately after escape form a new cyst. The organism 

 during temporary encystment does not undergo any internal 

 changes. It maintains its spherical shape and continues to 

 rotate. The second purpose is for reproduction by transverse 

 division. Likewise conjugation, except the early fusion of the 

 conjugants, may occur within the cyst. The third purpose is for 

 the reorganization of the cell. The cyst in this case is of the 

 permanent type, its walls being thick and dense. The individual 

 becomes quiescent, and loses its cilia and pharynx during internal 

 reorganization (Figs. 31-33). In this latter case the organism 

 can not be recovered from the cyst within a period of at least four 

 or more days or until its reorganization is complete. 



The micro-nucleus and contractile vacuole persist throughout 

 the reorganization without undergoing any appreciable change. 

 The macro-nucleus however passes through a distinct series of 

 changes as indicated in Figs. 31 to 34. Fig. 31 represents the 

 condition at the beginning of encystment when all of the organs 

 are present. The macro-nucleus is horse-shoe shaped, but in 

 many individuals it is spherical. In the first steps of macro- 

 nuclear metamorphosis the chromatin becomes ragged (Fig. 32), 

 with a clear border beneath the nuclear membrane. The central 

 region of the chromatin remains dark. Fig. 33 shows a third 

 stage in which the chromatin has taken a more spherical shape 

 with the ragged edges fraying out. Finally in Fig. 34 the macro- 

 nuclear change is complete, the more central portion of the 

 chromatin persisting as the new nucleus. The organs which were 

 lost have reformed. 



The macro-nuclear changes in P. griseus are similar to the 

 conditions found by Stolte, in Blepharisma undulans and by 

 Brand, in Vorticella microstoma and in Stylonychia mytilus. The 

 macro-nuclear activities during encystment are considered by 

 them as a cytoplasmic reorganization process since no new nuclei 

 are formed. According to Stolte cytoplasmic depression, oc- 

 casioned by adverse external conditions leads to encystment, and 

 nuclear depression leads to conjugation. In Didinium nastutum 

 however, as reported by Calkins and in Stylonychia pustulata as 

 21 



