16 F. M. BURNET 



If the reality of the eclipse phase is accepted, the phase of initial infection 

 of the cell could be defined as extending from first contact from the environ- 

 ment to the stage at which loss of morphological form and infectivity marks 

 the beginning of the eclipse phase. Experimental approaches to the study of 

 this phase are not as yet particularly effective, even when highly susceptible 

 and uniform host cells are used. They include: 



1. Direct examination of cell sections by electron microscopy. 



2. The rate and completeness with which virus is adsorbed by a known 

 area of susceptible cell surface. 



3. The susceptibility of attached virus to liberation by enzymes or inactiva- 

 tion by immune serum at various time intervals. 



4. The effect of soluble substances, e.g. mucoprotein inhibitors, in prevent- 

 ing adsorption. 



5. Modification of the susceptible cell surface by agents such as the 

 receptor-destroying enzyme (RDE) or metaperiodate ion. 



Some account will be given of the application of these approaches to (a) 

 influenza, fowl plague, and Newcastle disease viruses (NDV), and (b) polio- 

 viruses and other viruses of the intestinal group. 



Myxovirus. This is the only group of viruses for which some direct evidence 

 has been obtained by electron microscopy. Adams and Prince (1957) have 

 recently published pictures of NDV particles adsorbed to the surface of 

 "semisusceptible" cells of Elulich ascites tumor in mice. The virus particles 

 appear intact, except for the disappearance of the outermost coat of the 

 particle, perhaps as a result of having fused with or been "wetted" by the 

 cell substance. 



In the standard experimental situation of the allantoic cavity, it is usually 

 found that only 50-90 % of the inoculated virus is adsorbed before new 

 formation and liberation of virus commences. No fully satisfactory explana- 

 tion of this incompleteness of adsorption has been provided, but the presence 

 of potentially inhibitory mucoproteins in the allantoic fluid may well be the 

 most important factor. 



When attachment occurs, virus is temporarily removable by the action of 

 RDE and neutralizable by immune serum. According to Ishida and Acker- 

 mann (1956), the first step with PR8 and chorioallantoic membrane pieces is 

 to develop stability to RDE while still susceptible to inactivation by anti- 

 body. At 37°C, a second step rapidly follows by which immune serum 

 becomes ineffective. In rather similar experiments with Ehrlich ascites tumor 

 cells and the strain neuro-WS, Wagner (1955) found good adsorption of virus 

 at room or refrigerator temperature and easy liberation by RDE. With active 

 virus about 2 hours is required for union to become irreversible. When heat- 

 killed cells are used, virus remains indefinitely elutable by RDE, indicating 

 an active function of the cell in producing irreversibility. The possibility that 



