Sporozoa 303 



Gametogenesis and syngamy of Plasmodiida resemble these processes 

 in Eimeriida. The gametocytes (Fig, 6. 21, F, G) are more or less similar 

 in size, and are not associated during development. At maturity, the 

 microgametocyte rounds up and produces a few slender microgametes 

 which are rapidly separated from the residual protoplasm in a process of 

 "exflagellation." Syngamy results in a migratory ookinete (Fig. 6. 20, A-C). 

 This ookinete may pass through the gut wall of the vector and come to 

 rest beneath the membrane covering the gut; or as in Plasmodium circum- 

 flexum of birds and moscpiitoes (116), it may invade an epithelial cell of 

 the gut and develop there. The zygote apparently secretes a thin oocyst 

 membrane which is stretched as the parasite grows. Repeated nuclear 

 division results in a multinucleate sporont, or "oocyst" (Fig. 6. 22, A), 

 which produces many sporozoites. Sporoblasts are not formed, although 

 the sporont in Plasmodium becomes extensively "vacuolated" before 

 sporogony (Fig. 6. 20, F). Some workers have interpreted this apparent 

 vacuolation as a series of infoldings from the surface. The liberated sporo- 

 zoites (Fig. 6. 20, G) migrate through the tissues, and some of them reach 

 the anterior part of the digestive tract from which they are inoculated 

 into a vertebrate. 



After inoculation into a vertebrate, the sporozoites typically invade, or 

 are ingested by, phagocytic cells of the viscera within an hour or so. The 

 sporozoite becomes a trophozoite which develops into a multinucleate 

 schizont and undergoes merogony (Fig. 6. 21, A, B). The surviving mero- 

 zoites may enter other tissue cells, so that exoerythrocytic merogonic cycles 

 continue throughout the infection. In the Plasmodiidae, merozoites from 

 the first or a later exoerythrocytic merogony may enter red blood cells 

 and develop into schizonts, thus starting a series of cycles in the blood. 

 Sooner or later, merozoites develop directly into gametocytes which, at 

 maturity, are infective for the vector. In the Haemoproteidae, merogony 

 does not occur in erythrocytes, although gametocytes (Fig. 6. 22, E-J) 

 do invade blood cells after the infection has been in progress for some 

 time. 



For many years the erythrocytic stages of Plasmodiidae were the only 

 stages known in vertebrates. As a result of investigations by Cotilston, 

 Garnham, Hawking, Huff, James, Porter, Raffaele, Shortt, and others 

 (reviews: 32, 54, 55, 108), the occurrence of exoerythrocytic merogony in 

 reptilian, avian, and mammalian parasites is now clearly established. In 

 the typical infection, a pre-erythrocytic phase results from the devel- 

 opment of sporozoites introduced by the vector and may include one 

 (primates) or several merogonic cycles (birds). Products of the first 

 pre-erythrocytic merogony have been referred to as cryptozoites; the 

 merozoites formed in later exoerythrocytic cycles, as metacryptozoites 

 (56). Exoerythrocytic stages in later stages of an infection are known also 

 as phanerozoites {bl). Merozoites from pre-erythrocytic merogony con- 



