THE MORPHOLOGICAL APPROACH 75 



always clear. There is good evidence that the plaques contain the elementary 

 bodies within them (Bland and Canti, 1935; Rake and Jones, 1942; Weiss, 

 1949), but the origin of plaques is obscure. They are not always present even 

 in stained material (Burnet and Rowntree, 1935), and whether they contain 

 material excreted from the virus or products of cell reaction is not known. 

 For this reason the studies on the appearance of the virus during various 

 types of chemotherapy are particularly valuable. Weiss (1950) determined 

 the effect of penicillin and Aureomycin (Gogolak and Weiss, 1950) on embryo- 

 adapted feline and murine pneumonitis; in the embryonic cells they observed 

 that when the antibiotic "prevented the division of the virus particle" a 

 conglomerate of irregularly shaped plaques developed nevertheless. Vesicles 

 and elementary bodies developed apparently normally when the penicillin 

 was eliminated or destroyed. 



The exact mode of multiplication of these viruses in the cell is not yet 

 clear. The claim that they may multiply extracellularly (Weiss, 1949) does 

 not seem to be substantiated by data clearly free of artifacts. The extracel- 

 lular appearance might also be obtained because of the extreme thinness of 

 the alveolar walls (Karrer, 1956). A noninfectious phase (Girardi et al., 1952) 

 of the virus has been disputed (Bedson and Gostling, 1954). Electron micro- 

 scope studies of the early phases of virus multiplication have been negative 

 and have, therefore, supplied no useful data on this point. Light microscopic 

 studies have been equally unproductive. 



C. Fluorescent Antibody Staining 



In a study of the development of psittacosis virus in tissue cultures of 

 embryonic mouse liver, Buckley et al. (1955) found that virus antigen was 

 detectable as soon as one hour after the inoculation of the culture with large 

 amounts of virus. Focal accumulations of virus in the cytoplasm were 

 common under these conditions whereas, following inocula with small 

 amounts of virus, neither inclusions nor fluorescent antigens were detected. 



VI. Poxvirus 



A. Vaccinia 



Of the animal viruses, the poxviruses (Fenner and Burnet, 1957), with 

 vaccinia as the type species, are known in greatest morphological detail. 

 Vaccinia is one which has also been most completely identified. Since the 

 members of the group share a similar morphology, the process of infection 

 with vaccinia may be followed as an example. The cytoplasmic accretions of 

 differentially staining material, known since Guarnieri's original description 

 (1892), have been studied with increasing detail. By 1939 (Downie, 1939; 



