18 PROTOZOAN PARASITISM 



Just what is responsible for the phenomenon of 

 encystment is not known. In certain free living 

 forms it occurs when the liquid medium of the or- 

 ganism is drying. In certain forms which inhabit 

 the intestine it occurs in the lower parts, as the bowel 

 content becomes more solid. In certain cultures of 

 these organisms it has appeared to take place, in no 

 regular fashion, as the medium became inspissated 

 and concentrated. It has been recited above that 

 the writer had the occasion to observe encystment of 

 numbers of Trichovionas caviae in a medium of the 

 serum from its own guinea pig host, whereas the 

 change did not occur in similar medium from other 

 guinea pigs. 



Some of the cysts produced by the protozoa are 

 purely for protective purposes, and only the one 

 organism emerges from the cyst when conditions 

 again are favorable. In others, while the cyst is 

 protective, it is at the same time reproductive, and 

 the organism may divide into two or more offspring, 

 as the case may be, which may later emerge from the 

 cyst. 



In some protozoa encystment is a definite and regu- 

 lar part of the life cycle, essential to its continued 

 life. 



With most parasitic protozoa it is essential to the 

 continuation of the species, in that it is the particular 

 form in which they are able to survive exposure to 

 the external world and the vicissitudes of transfer- 

 ence to a new host. 



