82 ARTHUR C. UPTON 



GRAY: Before we leave contamination altogether, perhaps we should remember that 



Hewitt could transmit mouse leukaemia with one or two cells and he has even transmitted 



it by rubbing a cell suspension on the snout or ear of the animal without making any 



apparent lesion at all, so one has to be careful even at the cellular level. 



MOLE: If there was an agent that was infective naturally, and not just syringe-transmitted, 



there ought to be cage effects. Has anybody observed whether an animal that had been 



irradiated and kept with other animals could pass leukaemia on to them? 



COTTIER: This question is being considered in an experiment we are doing now. Our 



^"Co- irradiated animals are kept in cages of eight. Some cages have five animals dead 



from virus lymphoma — other cages have none. We wonder whether there is anything in 



this or not. But it is startling! 



