ANIMAL VIRUSES 491 



already adsorbed to the particles in the host cell. It is well known from 

 studies on vaccinia virus how difficult it is to remove enzymes from the 

 surface of virus particles once they have been adsorbed, 



C. Larger Viruses 

 1. Vaccinia Virus 



Vaccinia was the first animal virus obtained in a highly purified state 

 (see Smadel and Hoagland, 1942). It is bricklike in shape, with dimensions 

 -- 280 X 220 X 220 m/x (Williams, 1954; Peters, 1956) and a particle dry 

 weight r-^ 3.2 x 10' (Smadel et al., 1939 ; see Schramm, 19546). Associated 

 with these particles is a series of antigens: an NP (nucleoprotein) antigen, 

 an LS antigen, composed of a heat-labile (L) and a heat-stable (S) com- 

 ponent, an agglutinogen X, and perhaps another antigen responsible for 

 the formation of neutrahzing antibody (see Smadel and Hoagland, 1942). 

 The situation became even more comphcated when a hemagglutinating 

 principle was found in extracts of vaccinia infected tissue (hemagglutmin) 

 (Nagler, 1942; see Burnet, 1955a), which also is said to have a specific 

 antigenic structure (Chu, 1948b; Mayr, 1956). Indications of their chemical 

 nature have been obtained only for NP and LS antigens and for the 

 hemagglutinin. 



There is no doubt that the NP antigen is an intrmsic component of the 

 infective particle of vaccinia, since it is not fomid separate from the virus in 

 tissue extracts (Smadel ct ah, 1942). This is not the case for the LS antigen 

 and the hemagglutinin, which are present in relatively large amomits, 

 along with the infective particle, and hence may only be adsorbed to the 

 latter. However the available evidence has suggested to some authors that 

 the LS antigen is a part of the virus surface (see Smadel, 1952b). The 

 hemagglutinin has been practically completely separated from the virus 

 in several cases (Burnet and Stone, 1946; Chu, 1948a). 



The crude material for the purification of infective particles of vaccuiia is 

 obtained from scrapings of infected rabbit skin. The virus particles were 

 isolated from such material by repeated cycles of high- and low-speed 

 centrifugation. In this manner some 8-10 mg. of highly purified virus par- 

 ticles were obtained from several grams of rabbit pulp. Such virus pre- 

 parations had a single boundary in the analytical ultracentrifuge and by 

 electrophoresis. The analytical chemical data on different lots were very con- 

 stant, and did not change after further washing and centrifugation of the 

 preparations (see Smadel and Hoagland, 1942; see Smadel, 1952b). Overman 

 and Tamm (1956) recently showed that a single characteristic brick-shaped 

 particle is able to initiate an infection on the chorioallantoic membrane of 

 the egg. 



