PROTECTIVE AND REPRODUCTIVE CYSTS 49 



species of Colpoda multiply within cysts. The four daughter ciliates 

 usually rupture the cyst and escape (Fig. 38). On the other hand, they 

 may each become encysted within the primary cyst. The process of 

 encystment was probably first developed for purely protective purposes, 

 but various reproductive processes have become associated with it. It 

 must be remembered, however, that encystment is not essential to either 

 of these processes, as they frequently occur quite apart from any encyst- 

 ment whatever. 



In the majority of cases, when once formed, a cyst undergoes no change 

 in size or shape, though it may gradually increase in thickness. The 

 cysts of parasitic forms are ruptured or dissolved by the action of the 

 digestive fluids of a new host. In some cysts, however, there are formed 

 special pores which are kept closed by a plug of material which is more 

 easily dissolved than the rest of the cyst. Amongst the Sporozoa such 

 a pore in the oocyst is termed a tnicropyle, and through it the daughter 

 individuals which have been formed within the cyst emerge (Fig. 337). 



It sometimes happens that with growth of its contents the cyst in- 

 creases in size after it is first formed. This process is well illustrated by 

 the oocyst of the malarial parasite, which increases enormously in size on 

 the stomach wall of the mosquito (Fig. 391). A similar growth also 

 occurs in the case of the oocysts of the hsemogregarines {Hejpatozoon), and 

 the cysts of the flagellate (Prowazekella lacertw), which is parasitic in the 

 intestine of lizards (Figs. 253 and 254). It will be evident that if the 

 thickness of the cyst is to be maintained, there must be constant addition 

 to it of fresh material secreted by the enclosed organism. 



The cysts produced by any particular species of Protozoon are usually 

 fairly uniform in size, and possess distinctive features. On this account 

 their characters are of great importance for purposes of identification and 

 classification. 



THE PROTOZOAN NUCLEUS. 



GENERAL FEATURES. — The nucleus, which is an organized structure 

 containing chromatin, is the most important constituent of any Protozoan 

 cell, as, indeed, it is of all cells. It has been shown that the nucleus is 

 essential to the continuation of life, for individuals which have been 

 deprived of their nuclei, though they may survive and move about for 

 some time, quickly degenerate and die, while portions of the cytoplasm, 

 if they contain nuclei, often survive, regenerate, and continue their 

 existence. It seems probable that all the activities of the cell are governed 

 and regulated by the nucleus, which also appears to be mainly responsible 

 for the transmission of hereditary character. 



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