THE MORPHOLOGICAL APPROACH 101 



just look the same and may be equally essential to the mammary cell but 

 spread from mouse to mouse by a route other than the milk. Further com- 

 plexity was created when Fawcett and Wilson (1955) found somewhat larger 

 particles in hepatoma cells from the mice (milk factor C 3 H mice) which 

 commonly develop mammary tumors. Most of the work reviewed here on the 

 mammary tumor agent has been concerned with the comparative morpho- 

 logy of tumor cells and normal cells. There is such a dearth of knowledge about 

 living cells that Lasfargues's (1957a) cultures of adult normal and malignant 

 mouse mammary cells are of particular interest. His observation of particles 

 resembling those seen in the sections but found at the surface of the tissue 

 culture cells (Lasfargues, 1957b) adds to the potential importance of this 

 method of study. 



C. Warts (Human Papillomas) 



The viral etiology of these persistent benign growths has been indicated 

 for a number of years (van Rooyen and Rhodes, 1948). In any common 

 clinical sample of warts a few may be observed to differ from their neigh- 

 bors at the same sites in that they have a smooth margin, less keratinization, 

 and a surrounding erythematous inflammatory halo (Bunting et ah, 1952). 

 Water extracts of such warts were shown to have masses of small, round, 

 uniform particles which were often in a crystalline array (Straus et al., 1949). 

 Bunting showed that these apparent virus particles were closely packed 

 within the nuclei of affected cells and in such crowded conditions measured 

 about 38 nut in diameter. In the cells of the Malpighian layer of the neo- 

 plastic e2)idermis, the particles were in the nucleus and did not occur in the 

 prominent eosinophilic intranuclear inclusions or in the cytoplasm which 

 contained characteristic discrete dark masses. In cells forming the lower 

 stratum corneum of the wart, at a time when the nucleus was no longer 

 recognizable, the particles occupied almost the entire cell (Bunting, 1953a). 

 Bunting (1953b) later suggested that "apparently as the inclusion body 

 becomes larger, it becomes granular and then becomes 'converted' into virus 

 particles. These then increase in number and fill the nucleus. At first they are 

 not in an array, but when the number is so great as to distend the nucleus 

 when they are tightly packed then the array appears." Investigation of these 

 papillomas was suspended upon Bunting's untimely death, but the similarity 

 of these lesions with the changes wrought by some strains of adenovirus is 

 striking. 



D. Frog Adenocarcinoma 



Since the incidence of spontaneous carcinoma of the kidney of frogs may 

 be greatly increased by the injection of various filtrates from such tumors, 



