514 BIOLOGICAL EFFECTS OF RADIATION 



and capillaries, and phagocytes. There were occasional islands of 

 plasma cells and a few normoblasts. The substance of the spleen and 

 lymph nodes was greatly reduced and the germinal centers were visible 

 only as remnants. These organs contained a few round cells, many of 

 which showed mitotic figures. Large phagocytes enclosing brown pig- 

 ment and fragments of red blood cells were present in the marrow, lymph 

 nodes, and spleen. The persistence of these phagocytes in the presence 

 of total destruction of the lymphoid and myeloid systems suggests that 

 they are either very resistant to radiation, which is hardly likely, or that 

 they are formed after irradiation from local structures, probably from 

 the capillary endothelium. The clotting power of the blood was not 

 disturbed in these experiments. The hematocrit readings fell on the 

 average 10 per cent during the course of the experiment, the greatest 

 drop occurring within the last few days. 



If one or more vertebrae were not included in the field of radiation, 

 the animal recovered from the acute intoxication (334). In such cases 

 the circulating leucocytes did not drop below 500 white blood cells per 

 mm.^ and platelets were present in the blood smears at all times. These 

 animals slowly recovered from the leucopenia and survived. There was 

 a total lack of tissue and white-blood-cell reaction to any intercurrent bac- 

 terial or acute respiratory infection, probably because of the leucopenia. 



Localized irradiation of bone marrow or subtotal destruction of the 

 marrow is followed by active regeneration (36, 240, 335). Normally 

 fatty marrow may become erythropoietic (180). The irradiated marrow 

 cells (guinea pigs) showed changes quite early (335), consisting of degen- 

 eration and later complete destruction. These changes affected the 

 myeloid cells as well as the erythrocytic cells. The former were more 

 sensitive and regenerated less rapidly. Even after irradiation with 

 small repeated doses, which caused an increase of red and white cells in 

 the circulating blood, the histological findings in the bone marrow always 

 showed degenerative changes. Unirradiated bone marrow at a distance 

 from that irradiated showed changes in the morphology and distribution 

 of the cells. The megakaryocytes showed extensive nuclear injury, 

 the percentage of immature red cells was markedly increased, the red 

 cells predominated over the white, and the polymorphonuclears were 

 greatly decreased in number in the marrow, though if the doses were 

 small both these and the myelocytes remained normal (335). Among 

 the normal bone-marrow cells Casati (36) found that 50 days after 

 irradiation (rabbits) round nuclei predominated, either with or without 

 granules; there were only a few transition forms. There were also many 

 cells which made up a part of the reticulum that contained granules 

 similar to those of myelocytes and which might be interpreted as transi- 

 tion forms. They showed a tendency to develop toward the more 

 differentiated normal forms. These changes occurred in various grada- 



