266 DISCUSSION 



observed quite a striking population of macrophages, large pale cells, 

 particularly in the spleen, but I think also elsewhere, and they theorized 

 that these might be the principal operative cells. Have you seen these 

 in your animals ? We see them rather regularly in our runts following 

 neonatal injections of spleen cells, which means a mixture of different 

 morphological types, of course. 



Hildemann: We have not thoroughly evaluated our histological 

 sections, but we see no abundance of such cells in the spleen, nor in the 

 liver. 



Michie: I was most interested in Dr. Hildemann's account of the 

 weight-loss assay and look forward to its further development by Dr. 

 Hildemann and Dr. Russell. There are many purposes for which you 

 don't want to sacrifice your test animals and this is one potential assay 

 system where we can get round this disagreeable necessity. I have one 

 question about the magnitude of body weight depression which you 

 showed in your Fig. 3. In one of your shdes you had a photograph of a 

 couple of httermate mice, where one was in extremis and weighed about 

 5 g., whereas the other would weigh about 20 g.; differences of that 

 order are commonly encountered in the terminal stages, but the 

 magnitude shown on your graph seems to be of a much more modest 

 order. 



Hildemann: I am sorry, that was misleading. The mouse I showed 

 was an exceptional runt and a normal Httermate at 48 days of age. All 

 other severely affected runts died within 30 days after injection. I think 

 the reason for lack of greater disparity between the control and the 

 experimental values is simply that the worst affected animals die in 

 great numbers around two weeks of age. The experimental curve 

 gradually comes back up to meet the control curve at about 35 days as 

 the animals with sub-lethal disease recover. That is why I emphasize 

 the point that beyond 30 days in this system, the weight-gain assay no 

 longer distinguishes the experimental and control groups. The experi- 

 mental curve does not reflect weight loss, but rather the lack of weight 

 gain compared with the controls. The control curve was based on 

 some 500 normal mice including all of the Httermate controls in the 

 many Htters employed. 



Eichwald: In your organ-enlargement assay, could not your raised 

 Hver index and kidney index be due to a preservation of the normal 



