THE MORPHOLOGICAL APPROACH 81 



B. Histochemistry 

 Histochemical studies, including Feulgen stains, have shown a positive 

 reaction in the nuclei of infected cells (Crouse et al., 1950; Wolman and 

 Behar, 1952) and emphasize the need to study fairly early lesions 10 to 24 

 hours after the infection of the chorioallantoic membrane. It is not easy, 

 however, to determine the age of such lesions within individual cells, for 

 tissue culture studies indicate that virus is liberated from individual cells 

 from 6 to 12 hours after infection. Therefore, a typical pock results from 

 several "cycles" of virus liberation. A clear correlation of the development of 

 infectivity with the formation of a distorted nuclear pattern preceding the 

 classic inclusion in tissue cultures of rabbit corneal cells has been presented 

 (Scott et al., 1953). Scott's latest studies (Gray and Scott, 1954) on the titra- 

 tion of the virus in different fractions of chick embryo liver cells again support 

 the idea of localization within the nucleus. They propose that the necessity 

 of fractionating early in the development of the lesion might explain previous 

 failure to find nuclear localization (Francis and Kurtz, 1950). 



C. Electron Microscopy 



The clearest evidence for the intranuclear localization of the virus is the 

 electron microscope study of Morgan et al. (1954a). In this study of a chick 

 embryo-adapted strain of the virus, there is presented a series of beautiful 

 pictures, shadowed and unshadowed, of clumps and masses of particles, most 

 of which are found within the nucleus of the cell and which seem to have a 

 characteristic virus-like appearance. The authors refer to three size classes of 

 particles. The particles of the first class seem difficult to differentiate from 

 normal cell components, but those of the second class, seen clearly in un- 

 shadowed material, are uniform in size and have a dense center (40-50 m/x) 

 surrounded by a single membrane 70-100 nut in diameter. The particles of 

 the third class are formed in the cytoplasm; most of the particles show a 

 double outer membrane 120 to 130 m/x in diameter. The authors suggest that 

 there is a progression of development from the "primary" to the third stage. 



The intermediate-sized particles in the shadowed material in Morgan's 

 study are very similar to the "virus" particles found in 15 % of the tissue 

 cultures of normal chick embryos (Gey and Bang, 1951; Bang, 1954). Since 

 Morgan's strain of herpesvirus had been carried through a number of embryo 

 passages since 1947, the possibility of contamination by chick embryo 

 viruses needs to be considered. Grossly similar particles within the nucleus of 

 nerve cells infected with herpes have, however, been recently described 

 (Wyckoff et al., 1956). 



D. Herpes B 

 A related virus, herpes B, was studied by Riessig and Melnick (1955) in 

 tissue cultures of monkey kidney, and the cellular changes were correlated 



vol. in. — 6 



