LEUKAEMOGENESIS — VIRAL AND CYTOLOGICAL ASPECTS 73 



6. Removal of the thymus does not prevent induction of neoplasia in 

 other lymphoid organs (Ku'schbaum and Liebelt, 1955; Upton et al,, 1958) or 

 rid the thymectomized host of leukaemogenic virus (Miller, 1960). 



7. Irradiation causes an irreversible "initiating" or "priming" (Kaplan, 

 1961) effect in leukaemogenesis, which may be associated with the appearance 

 of detectable leukaemogenic "initiating" activity in tissues other than the 

 thymus (Berenblum and Trainin, 1961). 



8. The evolution of malignancy in thymic lymphomas may be gradual and 

 stepwise, as judged by serial biopsy and transplantation assays (Kaplan and 

 Hirsch, 1956). 



9. Passage materials derived from certain monomorphous mouse leukae- 

 mias contam a filterable agent, or agents, capable of eUciting neoplasia of 

 either lymphoid cells or myeloid cells, depending on the physiological 

 condition of the host and other factors (see Gross, 1961). 



10. The earliest spontaneous and radiogenic leukaemias require a sub- 

 stantially longer period for their development than those induced by highly 

 potent viral preparations. 



From these generalizations, one can only speculate about possible leuk- 

 aemogenic mechanisms. It would appear likely, however, that irradiation 

 may merely initiate a multistage process of leukaemogenesis by releasing or 

 activatmg a virus and that subsequent steps require the interaction of the 

 virus with susceptible host cells and possibly of host cells one with another 

 and with the host. The probability of completmg the neoplastic transforma- 

 tion would depend, according to such a hypothesis, on the number of virus 

 particles and susceptible host cells available for interaction. In such a 

 system, potent viral preparations would be rich in mature particles, and 

 promoting agents would mcrease interactions between particles and suscep- 

 tible cells and favour selection of altered cells. Thoughts, however, on the 

 possible nature of virus-host cell interactions in tumorigenesis (Latarjet, 

 1959; Dulbecco, 1961; and Kaplan, 1962) and on the role of radiation wiU 

 remaiti conjectural imtil further quantitative information becomes available, 

 particularly in the face of evidence that "non-oncogenic" viruses (e.g. 

 vaccinia, adenovirus, etc.) may exert a carcinogenic or co-carcinogenic 

 action (Duran-Reynals and Stanley, 1961; Martin et al., 1961; Trentiu et al, 

 1962). 



CYTOLOGICAL ASPECTS 



Karyotypic changes 



Since the time of Boveri (1912), oncologists have looked towards the 

 nucleus for insight into neoplastic transformation. Although techniques for 

 precise and detailed morphological analysis of mammahan chromosomes 



