100 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



more coiled polar filaments, typically enclosed in capsules, that 

 may be extruded under certain conditions. The spore wall often 

 is multicellular and this feature, among others, has led some 

 authorities to propose that the cnidosporidians are degenerate 

 metazoa. Electron-microscope studies include a brief observation 

 of whole spores o£~Plistophora by Steinhaus (1951) and two papers 

 concerned with species of the genus Nosema (Weiser, 1959; 

 Huger, 1960), parasitic in the fat bodies of insects. Both genera 

 belong to the Order Microsporidia, characterized by less complex 

 spores than many other cnidosporidians. 



Huger' s micrographs show that the Nosema spore wall has two 

 components: a thick, homogeneous inner layer and a lamellar 

 outer layer possibly derived from host tissues. The inner coat 

 thins at the site of the prospective pore. Within the wall is a low- 

 density anterior body with an apparent laminated structure. It is 

 not a true polar capsule; Huger calls it a polarblast and suggests 

 that it is a substance capable of swelling to cause extrusion of the 

 polar filament. The latter inserts in or near the prospective pore, 

 runs straight through the polarblast, then turns to the periphery 

 of the spore and spirals through as many as 18 turns. Its diameter 

 tapers from 230 m/z at its insertion to about 70 m/z at its free 

 posterior tip. It is enclosed in a double membrane and contains 

 longitudinal fibrils in a rather dense matrix. Because of this 

 density, observation of detail in the fibril bundle is difficult but 

 Huger notes a suggestive similarity to the organization of a 

 flagellum. The situation needs to be reexamined since there is 

 strong light-microscope evidence that the polar filament is a 

 tubule that everts upon extrusion (Kramer, 1960). 



Clusters of dense granules, probably polysaccharide, occur at 

 one side of the polarblast. The cytoplasm in the posterior part of 

 the spore is granular and too dense to reveal much internal 

 structure; two even denser nuclei are seen. 



