THE MORPHOLOGICAL APPROACH 97 



normal tissue. Thus, the cell types themselves cannot be evaluated. In spite 

 of this factor, the variety of lesions and the detailed photographic accounting 

 of them are unsurpassed. These authors not only confirmed Borel's findings 

 but described lesions of the nuclei in the basophilic cells in which an aggre- 

 gation of the chromatin was seen within a decidedly granular nucleus. Some 

 of the nucleoli assumed peculiar shapes or were pushed to one side by eosino- 

 philic masses negative to Feulgen stain. Occasional nuclei were packed with 

 eosinophilic inclusions. The marking-off of the central region of the cyto- 

 plasm was accompanied by the formation of intracellular crystals. Atypical 

 mitoses were common as was giant cell formation, and chromosome lag during 

 mitoses was illustrated. 



A phase microscopic study (Lo et al., 1955) of living cultured fibroblasts in 

 non-chicken media and inoculated with the Rous virus showed that normal 

 fibroblasts were transformed into abnormal cells which developed typical 

 paranuclear inclusions and fatty accumulations. Giant cells formed again 

 with great networks of normal appearing mitochondria. 



2. Electron Microscopy 



Since the first cautious description of "small bodies, the size of that esti- 

 mated for the transmitting tumor agents," having the appearance of ex- 

 traneous entities within chicken tumor cells in tissue culture (Claude et al., 

 1947), there has been a series of descriptions of these particles both in tissue 

 cultures (Bernhard et al., 1953; Epstein, 1956) and in sections of tumors 

 (Gaylord, 1955; Bernhard et al., 1956a). 



The identification of these particles as the tumor agent is as yet incom- 

 plete, but several of the criteria (Bang, 1955b) for such identification are 

 gradually being fulfilled. The particles from the first were recognized as 

 having an internal area of high density and a peripheral area of slight 

 density. Recent pictures show an external membrane and a thinner internal 

 membrane between the dense central portion and the external membrane 

 (Bernhard et al., 1956a). Thus, there is a fairly characteristic arrangement or 

 grouping of the particles and an internal morphology for the individual 

 particles. It is much more difficult, however, to make a clear statement relat- 

 ing these characteristic particles to the infectiousness of the material. Epstein 

 (1956) was able to show a correlation between the percentage of cells showing 

 these particles in vacuoles and the infectiousness of the extract. Whether 

 these round and vacuolated cells obtained from ascites-like passages of the 

 Rous tumor are indeed the tumor cells or are infected macrophages like those 

 originally described by Borel is not pertinent to the identification of the 

 particles. However, it is clear that there is a correlation between (a) the 

 cells which contain particles and (b) the infectiousness of the suspension; 

 and thus, indirectly with the particles. A more direct comparison of the 



VOL. III.— 7 



