1074 RADIATION BIOLOGY 



enhancing survival of mice exposed to 1025 r than earlier transplantation, 

 but it has not been determined when a state of irreversibility has been 

 reached. Actually, if establishment of a vascular supply to the trans- 

 plant is essential to the manufacture and transport of the factor (or 

 factors) in question, a conservative guess would be that supplying an 

 optimum amount of the factor to mice as late as 6 days after exposure to 

 1025 r would still significantly increase the survival. 



Administration of Suspension of Mashed Embryos. Intraperitoneal 

 administration of a suspension of 12-day-old mouse embryos is effective 

 in enhancing survival of mice exposed to 1025 r of total-body X irradia- 

 tion (Jacobson, 1952; Jacobson, Marks, and Gaston, 1951). This sus- 

 pension, prepared in the cold with or without the addition of normal 

 physiological saline or buffered saline, when given intraperitoneally in a 

 dosage of from 0.5-1.0 ml, 2-6 hours after irradiation of the recipient, 

 results in 30 per cent survival. Suspensions of baby or adult spleens 

 prepared and administered in a similar manner have thus far been ineffec- 

 tive in enhancing survival of mice exposed to 1025 r but have been effec- 

 tive in enhancing the survival of mice exposed to 800 r (Jacobson, 1952; 

 Jacobson, Marks, and Gaston, 1951). Chick embryo suspensions (age 

 of embryos, 11-14 days) prepared and administered in a similar manner 

 to the mouse embryo suspensions have been reported by Marks (unpub- 

 lished data, 1952), Stroud and Brues (unpublished data, 1952), and 

 Jacobson (1952) to be ineffective in enhancing survival of mice exposed to 

 X radiation in the LD 5 o range or above. 



The factor (or factors) in the embryo or spleen suspensions responsible 

 for recovery from radiation is probably the same as that responsible for 

 the effectiveness of the spleen shielding and spleen implants. Two 

 possible explanations for the effectiveness of cell suspensions are obvious: 

 (1) that the cells in the suspension quickly implant and begin elaboration 

 of the factor or factors effective in initiating tissue regeneration through- 

 out the body or (2) that the peritoneal cavity serves as an incubator that 

 allows the cell suspension to remain alive and to elaborate the factor 

 responsible for increased survival and tissue regeneration in irradiated 

 mice. 



Homologous Bone-marrow Injection. Lorenz et al. (1951) have shown 

 that, although 900 r is the LD 99 for genetically homogeneous hybrid 

 LAFi mice, approximately 75 per cent survive this dose if bone marrow 

 aspirated from the long bones of normal nonirradiated mice of the same 

 strain is injected intravenously within an hour after the exposure. If the 

 bone marrow is administered intraperitoneally, survival is slightly less 

 (ca. 50 per cent). The author estimates that the total weight of the 

 injected marrow is approximately 1.5 mg. The recovery of the hemato- 

 poietic tissue of the bone-marrow-treated mice, as in the spleen-shielded, 

 spleen-implanted, or embryo-suspension-injected mice, is hastened. 



