THE MORPHOLOGICAL APPROACH 77 



2. Electron Microscopy 



The application of the electron microscope to a study of thin sections of 

 these infections (Bang, 1950; Morgan and Wyckoff, 1950) rapidly led to a 

 whole new series of morphological findings (Fig. 3). The elementary bodies, 

 which had been shown to have a dense central portion (Green et al., 1942; 

 Dawson and McFarlane, 1948), and which have been extensively analyzed by 

 enzymatic digestion methods (Peters and Stoeckenius, 1954), were readily, 

 identified in the tissues (Bang, 1950; Gaylord and Melnick, 1953; Wyckoff, 

 1951). Their fine structure has been analyzed in detail (Fig. 4) by Morgan 

 and co-workers (1954b). In several studies (Gaylord and Melnick, 1953; Dohi, 

 1956; Matumoto and Dohi, 1956), large homogeneous inclusions with sharp 

 edges have been found in the cytoplasm of the cells. In some cases, e.g., 

 vaccinia in cornea (Gaylord and Melnick, 1953) and ectromelia in ascites cells 

 (Dohi, 1956), the evidence seems clear that these inclusions lack any discrete 

 particles within them; in others, there is clearly a mixture of formed elemen- 

 tary bodies or viruses and a homogeneous material. Finally, it is clear that, 

 as in the epithelial cells studied by Bland and Robinow (1939), the virus may 

 also be diffusely distributed throughout the cytoplasm. The elementary 

 bodies of ectromelia have been found to be nicely delineated in phase micro- 

 scopy of Ehrlich ascites cells infected with this virus (Matumoto and Dohi, 

 1956). The typical large cytoplasmic masses, seen particularly in infections 

 with ectromelia, are probably identical with the intensively eosinophilic 

 masses seen with the light microscope in cowpox (Downie, 1939) and ectro- 

 melia (Downie and McGaughey, 1935). However, in such a spot as the chorio- 

 allantoic membrane these must be clearly differentiated in the electron micro- 

 scope from red cells which may have been manufactured in situ or may have 

 been phagocytized, as in ascites tumor cells. The nature of these homo- 

 geneous bodies, which in most cases clearly are not red cells, would be of 

 considerable interest. 



Secretion of these masses, perhaps as waste material, by the cell in reaction 

 to the altered metabolism induced by the virus is the most likely explanation 

 of their existence. Such a possibility is supported by Matumoto's (Matumoto 

 and Dohi, 1956) claim that two strains of ectromelia when growing within 

 ascites tumor cells differed from each other in their relation to these "blue 

 staining inclusion bodies" which in the one strain formed independently of 

 the virus, and in the other were formed with a number of viral particles 

 enclosed within. 



The continuing electron microscope and enzymatic analyses by Peters 

 (1957) of separated virus particles emphasizes that the central portion of the 

 virus particle is not uniform. He describes it as ring-shaped. The varying 

 fine morphology (Gaylord and Melnick, 1953; Morgan et al., 1954b; Eaves 

 and Flewett, 1954), as seen in thin sections has not yet been clearly related 



