THE CAUSE AND NATURE OF EXCYSTMENT. 53 



From this he concludes that hydrogen-ion concentration is 

 probably one of the most important factors in producing encyst- 

 ment. It seems to us that he would have been justified in 

 concluding that adverse hydrogen-ion concentration might 

 slightly speed up the process, if the internal condition of the 

 protozoan disposed it towards encystment. It is just such 

 conclusions as Koffman's that have led to an over-emphasis of 

 the importance of adverse environmental factors in producing 

 encystment and have led to some authors speaking of the 

 phenomenon as a "passive adaptation to adverse environment." 

 Although that may be true in a few instances, a critical review 

 of the literature certainly does not indicate that it is the general 

 rule. 



The Nature of Encystment. In a former paper Kater has shown 

 that extensive changes occur during the encysted stage of 

 Polytomella citri. This does not particularly involve the nucleus, 

 the budding off of the new centriole and basal granule from the 

 karyosome being the only possible regenerative change that was 

 found. The most striking changes take place in the cytoplasm. 

 The starch rapidly disappears when this resting phase is entered, 

 and, coincident with its disappearance, the metachromatic 

 granules are formed in great abundance. These are absorbed 

 before excystment occurs. 



Before attempting a discussion of the meaning of encystment 

 in this flagellate, we must decide why prevention of encystment 

 for several months results in loss of the tendency to store starch, 

 and, consequently, to encyst. On a priori grounds we can say 

 that it is either due to selection, direct effect of the environment, 

 or a combination of the two. Although we may not be able to 

 answer this conclusively, the present evidence indicates that 

 selection does not enter into the matter. If it were due to 

 selection of an encysting strain we should not expect cysts taken 

 from 3 and C"j to give rise to cultures that would encyst in 

 exactly the same time, two days, yet such is the case. Cj 

 encysted much more quickly than did 3 and, if there were 

 anything in the strain, the offspring should behave likewise. 

 In the second place, Fi showed only a very slight tendency to 

 encyst, mostly dying. Since these cultures were seeded from 



