DISCUSSION 399 



Miller: This is what we are trying at the moment. 



Hildemann: I was most interested in Dr. Miller's finding that im- 

 munologically unresponsive thymectomized mice showed a marked 

 deficiency of lymphocytes in peripheral blood. In this connexion, in 

 studying the ontogeny of leucocytes in newly-hatched bullfrog tad- 

 poles, we have found that all definitive types of leucocyte appear 

 during an age when animals may be made completely tolerant toward 

 skin homografts — with one exception, small lymphocytes, which 

 appear at the very time (40 to 50 days post-hatching) when the tadpoles 

 develop the capacity to reject skin homografts. Now admittedly this is 

 just circumstantial evidence for the function of these small lympho- 

 cytes, but it is of interest that these results are quite consistent with your 

 view that the unresponsiveness you have observed is associated with the 

 paucity of lymphocytes. 



Loutit: I wonder if there is anything in your evidence which would 

 suggest that the thymus is not a half-way house for cells on their way 

 from the bone marrow to the lymphoid tissue. I have some speculations 

 in press (Loutit, J. F. [1962]. Irradiation of Mice and Men. Chicago: 

 Chicago University Press) which your work may show to be wrong. 

 The speculations were based on results obtained by my colleagues. 

 Ford and Micklem, in the adult mouse, irradiated lethally and then 

 restored with bone marrow, syngeneic or allogeneic (preferably syn- 

 geneic) with a marker. When the mouse recovers, the marker appears, 

 not only in the bone marrow, but in the thymus and in the lymphoid 

 tissue. Ford and Micklem (unpublished) have given similar mice syn- 

 geneic marked bone marrow plus marked lymphoid cells. The lym- 

 phoid cells' marker appeared only in the lymph nodes, not in the 

 thymus nor in the bone marrow; but the bone marrow cells' marker 

 appeared in the bone marrow and the thymus. That is the early finding. 

 After a month or two, the marked lymphoid cells disappeared from the 

 lymphoid tissues and the bone-marrow marker appeared throughout. 

 So that it looks as if the lymphoid cells repopulate the true lymphoid 

 tissues initially but not permanently. The bone marrow, we considered 

 from this evidence, contained polyvalent stem cells which repopulated 

 the bone marrow, and the bone marrow repopulated the thymus. 

 Cells of the marrow that were differentiating towards lymphoid cells 

 probably circulated to the thymus, had a temporary period of residence 



