VITALITY 267 



Gregory (1925) for example has shown that salts and change of 

 medium are deleterious to very young forms of Uroleytus mobilis 

 while older forms are not affected. This is in line with Child's 

 results in connection with the action of potassium cyanide on many 

 kinds of organisms, those parts which have the highest metabolic 

 rate being first to succumb. 



The differentiations indicated above are physiological in nature 

 and are rather intangible. Other differentiations characteristic of 

 youth while also physiological are indicated by morphological or 

 structural modifications. Of these the most noteworthy are the 

 different types of cysts which are secreted by all kinds of Protozoa. 

 Some are temporary cysts in which no endomictic phenomena occur 

 (e. g., division cysts of Colpoda, Tillina and many flagellates). Ex- 

 perimentally produced cysts are presumably of this kind (see 

 Lwoff, 1927; Wolff, 1927; Garnjobst, 1928; Bresslau, 1921, etc.). 

 Encystment has been generally regarded as a means of protection 

 for the organism against adverse conditions of the environment. 

 This is probably more traditional than accurate, for very few Pro- 

 tozoa are actually known to encyst when the external conditions 

 are unfavorable. Mast (1923) for example finds that food and 

 temperature have little effect in causing Didinium nasutum to 

 encyst, but encystment takes place under the best conditions. It 

 is more probable that organisms which have had the power to 

 encyst persist under such conditions while the great majority are 

 killed. Cutler (1919), however, gives evidence to show that skatol 

 induces encystment in Endamoeba dysenteriae, and Cleveland (1927) 

 that encystment of Paramecium occurs when injected into the rec- 

 tum of frogs. This power to form reorganizing and "permanent" 

 cysts appears to be a factor of young organisms induced possibly, 

 as Mast (1923) suggests, by the accumulation of waste materials. 



The sporoblast capsules of all Sporozoa, with the exception of the 

 Cnidosporidia (p. 552), are formed as a result of the first activities 

 of the young fertilized cell and they do not occur again. The same 

 phenomenon is characteristic of zygotes in Sarcodina. With Infu- 

 soria where fertilization is accomplished through conjugation such 

 zygote cysts are practically unknown, but encystment, with reor- 

 ganization processes, is possible during the early period of the life 

 cycle until maturity, when it is apparently replaced by conjugation. 

 Thus in Uroleptus mobilis in connection with which this phenomenon 

 has been carefully studied, encystment may occur within three 

 days after fertilization but usually after a longer period has elapsed. 

 Such encystments occur under the same external conditions as do 

 conjugations later in the cycle. So-called "conjugation tests" arc 

 made every week or ten days. For these, all of the individual cells 

 of a series left over a daily isolation has been made are placed in a 

 large container with fresh medium. Here they are allowed to 



