1076 RADIATION BIOLOGY 



changes in the blood and blood-forming tissues of the irradiated mouse 

 were less conspicuous than those in a single mouse similarly irradiated, 

 while in the nonirradiated parabiont, the changes were only slight. More 

 recently Brecher and Cronkite (1951) reported that approximately 

 50 per cent of rats exposed to 700 r of total-body X irradiation survive 

 if they are joined to normal nonirradiated litter mates within a few hours 

 after irradiation. None of the controls given the same does of irradiation 

 survived the 28-day period of observation. Hematopoietic regeneration 

 was more rapid in the irradiated rats with a parabion than in the irra- 

 diated controls. These findings, like those of Lorenz et al. (1951), 

 corroborate Jacobson's previously reported observation that effective 

 postirradiation "therapy" is a reality. Like the embryo-suspension 

 experiments described, however, the experiments of Brecher do not point 

 out the cellular source nor the identity of the effective factor (or factors) 

 concerned. 



Relation between Quantity of Shielded or Implanted Splenic Tissue and 

 Effects on Survival. Two separate observations indicate that a definite 

 relation exists between the quantity of implanted or shielded tissue and 

 the effect as measured by hematopoietic regeneration or survival from 

 irradiation (Jacobson, Simmons, Marks, Gaston, et al., 1951). 



1. The transplantation of two spleens (weight, ca. 5 mg) from 7- to 

 12-day-old baby mice into the peritoneal cavity of mice immediately 

 after irradiation (1025 r) does not enhance survival, although the trans- 

 plantation of four spleens (weight, ca. 10 mg) is effective in reversing the 

 process in time to allow recovery of the animal (Jacobson, Simmons, 

 Marks, Gaston, et al., 1951). 



2. In the regular spleen-shielding technique the main splenic pedicle is 

 left intact, but a small blood vessel at the distal tip of the spleen is 

 severed to facilitate exteriorization and lead shielding of the spleen. 

 Invariably from one-fourth to one-half the spleen proximal to the severed 

 vessel becomes infarcted and undergoes liquefaction necrosis. If this 

 vessel is not cut and the whole spleen is shielded and remains intact, 100 

 per cent of the animals survive exposure to 1025 r rather than the expected 

 77 per cent. In fact, with a total-body exposure to 1300 r, only 3.4 per 

 cent of the animals survive if the distal vessel of the spleen is cut during 

 the shielding procedure, whereas 26.9 per cent survive this exposure with 

 spleen shielding if the distal vessel is not cut (Jacobson, Simmons, Marks, 

 Gaston, et al., 1951). Hematopoietic regeneration is complete by 8 days 

 in the animals exposed to 1300 r with spleen shielding and in which the 

 distal vessel to the spleen is not cut. In the animals, exposed to this 

 dose with spleen shielding but with the distal vessel cut, regeneration is 

 not as rapid (Figs. 16-19, 16-20). This observation like observation 1 

 indicates that the quantity of the factor being produced is directly related 



