54 I. BEEENBLUM AND N. TRAININ 



because from the bone-marrow I speculate that cells might aid the regeneration of the 

 thymocjrtes in the cortical layer of the thymus. But if, on top of the irradiation, we add 

 the urethane, then tliis bone-mattow in the host animal is more depleted and more 

 damaged and caimot repopulate the thymus. This damages the thymocytes much more 

 excessively and the regenerated cells then undergo malignant transformation. I believe 

 this is the role of urethane. 



BERHNBLTJM: There are several points which argue against this. One is that if you give 

 urethane plus excess bone-marrow, you stiU get your leukaemogenesis. 

 roller: I asked you when you gave your bone-marrow and you said you gave it 24 

 hours after the irradiation. 



berenbltjm: Five times, after each urethane injection, which, according to Kaplan's 

 experience, seems more than adequate. We gave a great excess of bone-marrow 

 suspension. If we had given the bone-marrow after a shorter interval than 24 hours, then 

 the criticism would have been made that the urethane had destroyed the bone-marrow. 

 ALEXANDER: You Say that 98% of the urethane is no longer there as urethane. To explain 

 the multifarious activity of urethane one has to have some. . . . 



berenblum: When I say that 98% of urethane is gone, I mean of urethane and/or 

 its breakdown products; because the way this was tested was by giving ^*C labelled 

 urethane and then tracing the ^*C. The i*C has disappeared from the animal to the 

 extent of about 98% within 24 hours. 



ALEXANDER: I still find it difficult to believe that aU the activity of the urethane carbon 

 is gone, because when one gives urethane for the alleviation of leukaemia in man then 

 the activity is manifested over very long periods after the drug has been discontinued. 

 BERENBLtTM: The effect of the damage to most cells may go on perhaps for years. I'm 

 talking of the action itself — the action in the active sense. The consequences of urethane 

 action depend on what kind of damage is done. 



LAMERTON: You may in fact have prevented the regenerating capacity of your bone- 

 marrow with urethane or produced conditions which don't allow the regeneration. 

 BERENBLUM: We have a number of experiments in progress to attempt to answer this 

 question. In some of these, the two-stage process is being investigated in relation to 

 thymectomy, performing the thymectomy operation at different stages in the various 

 groups, in order to determine at which stage the removal of the thymus becomes critical. 

 In other experiments, involving the transfer of tissues from irradiated mice and adminis- 

 tering urethane to the recipients, the influence both of normal bone-marrow injections 

 and of thymectomy are being tested, both on the donors and on the recipients. From 

 all these experiments, an answer should eventually be forthcoming. 

 ROLLER: When do you do the thymectomy? Miller's experiments show that the thymus 

 has done all the damage already, one week after birth. You must therefore remove the 

 thymus one day after birth. 



COTTIER: If ,you transfer brain, did you perfuse this brain first or did you not? What is 

 the important thing — the blood, the blood cells or the plasma? 



BERENBLTTM: We uscd total brain mince. In the second experiment, we again used total 

 brain tissue, irradiated in vitro. In the third experiment, we irradiated the animal; took 

 out the brain; centrifuged the suspended mince at slow speed to get rid of most of the 

 deposit and then gave four consecutive high-speed centrifugations at 7,000 X ^ to 

 obtain what is effectively a ceU-free extract. There is a further experiment in progress 

 which is even more critical, using filtered material. This experiment has only been going 

 six weeks. 



COTTIER: I have been wondering where the active agent would be located. You mentioned 

 brain, lung, spleen. Now lung ordinarily contains lymphoid cells, bram to my knowledg 



