POSTIRRADIATION TREATMENT OF MiCE AND RaTS 151 



have seen the reappearance of mitoses of murine cells. It is, 

 however, premature to do more than record the findings so far. 



In preliminary experiments where less than the 99 per cent 

 lethal dose of 950 r has been given to CBA mice, we see 

 this reversal of cellular types at a comparatively early time 

 after irradiation. When CBA mice are given 545-575 r and 

 treated with T6 spleen, the regenerating tissues are mainly 

 of the T6 type in the first week, but later belong mainly to 

 the normal CBA type. In further preliminary experiments 

 CBA mice were irradiated over only part of the body. The 

 hind third was given 1200 r and T6 spleen was then injected 

 intravenously. The regenerating bone marrow of the femora 

 and inguinal lymph glands at 5 days corresponded in cell- 

 type to the normal CBA host. 



The interpretation of these experiments is clear. In the 

 mouse given the LD99 of X-rays the regenerating cells, seen 

 in mitosis, of the haemopoietic tissues are almost without 

 exception characteristic of the material from the donor. The 

 living cells in the preparation injected must, therefore, be 

 dividing and colonizing the empty spaces of bone marrow and 

 lymphatic tissue. The alternative explanation of the former 

 experiment of Mitchison — that the host had incorporated 

 antigens — can no longer be maintained. In the homologous 

 transfer it is inconceivable that the host's cells had accepted 

 whole chromosomes (translocations at that), rejected some of 

 its own to maintain a normal complement and still had a 

 balanced set for division. Equally it is unnecessary in the 

 heterologous transfer to postulate complete exchange of 

 chromosomes by the host. 



The preliminary results of experiments involving sub- 

 lethal doses of irradiation suggest that the length of the 

 symbiosis of donor's and host's cells may be dependent on the 

 dose of radiation given. It seems that the immune mechan- 

 isms which normally determine compatibihty of tissue grafts 

 are in abeyance following massive doses in the lethal and 

 supralethal range. This refractoriness is long lasting. From 

 the direct evidence of Table III, we show it has lasted for 



