POLYMORPHISM AND LIFE-CYCLES 167 



which it multiplies to form a number of germs, which may or may 

 not be enclosed in sporocysts, in different species. Cysts and spores 

 pass out of the host, and do not develop further unless they are 

 devoured by a second host of a species in which they are able to 

 establish themselves ; if this event takes place, the spores germinate 

 in the new host and produce a fresh cycle of infection, each germ 

 when set free growing up into a schizont. In this case it is seen 

 that the schizonts represent the multiplicative, the sporonts the 

 propagative, phase, and that in the latter resistant cysts are pro- 

 duced as a protection against the vicissitudes of the outer world, 

 to which the parasite must expose itself during this phase of its 

 life-history. 



An example of a parasite which infects two distinct species of 

 hosts in the course of its life-history is furnished by the malarial 

 parasites (p. 360, Fig. 156). In this case there are first of all 

 schizonts which, like those of Coccidium, reproduce themselves by 

 multiple fission, this part of the life-cycle being passed in the blood 

 of a vertebrate host. Later, sporonts are generated which under 

 normal circumstances are incapable of multiplication in the verte- 

 brate host, or, indeed, of any further development, unless taken up 

 by another host, in this case a mosquito, which takes them from 

 the vertebrate host by sucking its blood. In the stomach of the 

 new host the sporonts behave in a similar manner to those of 

 Coccidium that is to say, they give rise as gametocytes to gametes, 

 which by syngamy produce zygotes. The zygotes grow and repro- 

 duce themselves by multiple fission, forming an enormous number 

 of minute germs or sporozoites, which do not develop further unless 

 they pass from the mosquito back into the blood of a suitable 

 vertebrate host, in which they start a fresh developmental cycle. 



The life-cycle of the malarial parasites shows that a given phase 

 of a parasite is only to be regarded as multiplicative or propagative 

 in relation to a particular host. In the vertebrate blood the 

 schizont is the multiplicative, the sporont the propagative, phase. 

 As soon, however, as the sporont passes into the mosquito, it becomes 

 there the multiplicative phase which gives rise ultimately to the 

 sporozoites, representing the propagative phase in the mosquito. 

 The sporozoites in their turn, when they reach the blood of the 

 vertebrate, develop there into schizonts. Thus one and the same 

 stage in the life-cycle represents one phase in one host and another 

 in another, according to circumstances. It should be noted further 

 that in the life-cycle of the malarial parasites resistant cysts are 

 unnecessary, since the parasite never comes out into the open, but 

 passes the whole of its existence in one or the other of its two hosts ; 

 consequently such cysts are not formed at any stage of the life- 

 cycle in these forms. 



