490 BIOLOGY OF THE PROTOZOA 



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

 materials. In a relatively few cases, however. Protozoa will encyst 

 as a preliminary to reproduction by division {Tillina magna, 

 Colpoda, etc. among the ciliates, and in some flagellates). 



The sporoblast capsules of all Sporozoa with the exception 

 of the Cnidosporidia (p. 451), 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 and 

 Mastigophora. With Infusoria where fertilization is accomplished 

 through conjugation such zygote cysts are practically unknown, 

 but encystment, with reorganization processes, is possible during 

 the early period of the life cycle until maturity, w^hen it is apparently 

 replaced by conjugation. Thus in Uroleptus mohilis 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" are made every week or ten days. For these, 

 all of the individual cells of a series left over after a daily isolation 

 has been made are placed in a large container with fresh medium. 

 Here they are allowed to accumulate until thousands of individuals 

 are present. The food medium is not replenished and such mass 

 cultures are watched daily until the individuals die. After five 

 or six days conjugations will take place provided the organisms 

 are mature; if they are not mature encj'stment takes place and it 

 frequently happens that thousands of cysts are present in one 

 container. From the records made during the last seven years it 

 is possible to work out the incidence of encystment and of conjuga- 

 tion for each series. Fig. 204 shows the vitality curve of ten different 

 series. The periods of the first encystments observed and the last 

 encystments in the different series are connected by vertical lines. 

 The first appearance of conjugation is indicated in the same manner 

 but with double lines. In some series it happens that both encyst- 

 ments and conjugations occur in the same container but tests of 

 the same series made later give only conjugations. With Uroleptus 

 at least it appears therefore that encystment is a characteristic 

 phenomenon of young organisms comparable with the Dauersporen 

 of phytoflagellates, and lower plants generally, after fertilization; 

 and that the powder to encyst disappears with the advent of maturity. 

 It is highly desirable to have similar data for other types of ciliates; 

 some incomplete studies on Didinium nasntuin indicate analogous 

 phenomena (Calkins (1915). 



B. Cyclical Differentiations Peculiar to Old Age.— Toward the 

 end of the life cycle even more characteristic differentiations occur 

 than at the outset. In many cases these are coincident wuth the 

 fertilization phenomena and will be discussed in connection with 



