604 



PROTOZOOLOGY 



of P. cynomolgi, and when mature, give rise to 800-1000 merozoites. 

 Thus exoerythrocytic stages and development have definitely 

 been demonstrated for Plasmodium in various host groups, although 

 morphological and developmental details, distinction between them 

 and other little known organisms such as Toxoplasma (p. 625) and 

 interrelationship between them and erythrocytic stages, had to be 

 looked for in future investigations (Fig. 258). General review of E.-E. 

 development (Huff, 1947, 1948; Garnham, 1948). 



Fig. 258. Diagrammatical life-cycle of an avian Plasmodium (Several 

 authors). Well established phases are connected by solid lines, while 

 undetermined and recently suggested phases are indicated by broken 

 lines, a, sporozoite injected into host bird by a mosquito; b-e, exoerythro- 

 cytic schizonts and schizogony in monocytes; f-i, commonly seen schi- 

 zogony in erythrocytes; j, macrogametocyte; k, microgametocyte. 



The incubation period of Plasmodium infections in man varies due 

 to various factors such as the strain, vitality and number of the spo- 

 rozoites injected by the mosquitoes, the varied susceptibility on the 

 part of host, etc. Boyd and co-workers found that the incubation 

 periods for the three species of humaD Plasmodium which they stud- 

 ied were, as follows: In P. vivax. 8-21 days (the majority 11-14 days) 



