612 Malaria 



the epithelium and rounds up beneath the outer layer of the mid-gut 

 within a period of 24-48 hours. Here the zygote begins to grow, apparently 

 enclosed in a thin "oocyst" membrane. Several to many oocysts may 

 develop in a single mosquito, usually without any appreciable effect on 

 the host. The rate of growth varies with the species of Plasmodium and 

 Anopheles and among different oocysts in the same mosquito, and is 

 influenced also by the external environment. Oocysts of P. vivax may 

 reach a diameter of 50[ji, or more in 1-2 weeks under favorable conditions. 

 Oocysts of P. falcipanitn reach a comparable size in two weeks or so, 



/• '" \ 



I 







■^::.::j 



B 



i;,' 



Fig. 13. 6. Development of a malarial parasite in the mosquito; dia- 

 grammatic: A. Microgametogenesis, or "exflagellation.'" B. Macrogamete. 

 C. Ookinete. D. Zygote encysted on the wall of the stomach. E. Multinu- 

 cleate oocyst some time ijefore the formation of sporozoites. F. A sporo- 

 zoite. 



while those of P. malariae grow a little more slowly (69). During growth, 

 rapid nuclear division occurs and sporogony finally produces thousands 

 of sporozoites. Eventual rupture of the "oocyst" releases the sporozoites 

 into the tissue spaces and some of them reach the mouth-parts of the 

 mosquito. 



The time required for completion of the mosquito phase varies con- 

 siderably with environmental conditions. Under comparable laboratory 

 conditions, P. vivax and P. falcipanwi have required less than three 

 weeks and P. malariae about four (70), but these periods are shorter in 

 more favorable environments. 



The sporozoites of P. vivax (25) and presumably the other species 



