92 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



under study by Beams and Anderson (unpublished). These 

 organisms undergo an attached phase during which they are 

 partly embedded in the cells of the host gut. Electron micro- 

 graphs (Fig. 31, PI. IX) indicate a very intimate association 

 between host and parasite cell membranes. The younger parasites 

 lack pellicular ridges ; these begin to develop on the free posterior 

 surface of the body, where it protrudes from the host cell. Internal 

 differentiations are minimal; the septum dividing anterior and 

 posterior body regions is not visible, ectoplasm and endoplasm 

 are not distinct, and no myonemes or fibrillar structures can be 

 detected. 



Most of the authors studying gregarine fine structure have been 

 concerned with the mechanism of the eerie creeping movement of 

 the adult cells. Body movements — flexion and twisting — are 

 common, but gliding occurs in the absence of any detectable 

 change in body shape. It is accompanied by the production of a 

 conspicuous mucus trail, and this has prompted the suggestion 

 that a sort of mucus jet propulsion operates. The consensus 

 among recent students (Kummel; Beams et a/; Jarosch, 1959) is 

 that mucus expulsion alone is not adequate to explain movement, 

 but rather that a submicroscopic contraction of cortical structures 

 causes local displacements of the body surface, perhaps against 

 the mucus substrate. Kummel, studying living cells, noted the 

 simultaneous passage of particles over the body surface at different 

 rates, independent of particle size, or in different directions. Given 

 the semi-rigid longitudinal crests, it is conceivable that waves of 

 contraction in cortical fibril systems or of minimal shortening of 

 the circular myonemes, even though not visible at the light- 

 microscope level, could result in body displacement. 



For obvious reasons, the malaria parasite, Plasmodium, has been 

 far more extensively investigated than any other sporozoan. 

 Asexual generations occur in reptiles, birds, and mammals, sexual 

 reproduction in mosquitoes. Sporozoites, inoculated into the 

 vertebrate host by the bite of the insect, first enter phagocytic 

 cells of the reticulo-endothelial system, and undergo one or more 

 exoerythrocytic reproductive cycles, each mature cell producing 

 many daughters called merozoites. The latter invade either more 

 cells of the same type or erythrocytes. In the latter case repeated 

 erythrocytic cycles follow, and eventually some of these merozoite 



