L. H. GRAY 



that the lowest values of the aberration coefficient observed in vivo agree 

 closely with the anaerobic in vitro value, and that the highest value observed 

 in vivo, in animals injected with hydrogen peroxide, are close to the maximum 

 fully oxygenated in vitro value. We think it probable that in vivo and in vitro 

 sensitivities are also equal at all intermediate oxygen tensions, and that the 

 scatter of the points in Figure 4 represents the extent to which the average 

 oxygen tension in the fluid of the cavity lies below that measured by the 

 bubble, which will inevitably be located adjacent to one of the vascularized 

 membranes. 



If there are nx 10^ cells/ml. in the peritoneal cavity, the oxygen consump- 

 tion of the fluid will l)e 1 1 n [xl. 02/g/min*,or 0-66 ;? [jil./mg wet weight/hour. 

 If this fluid were stationary and in contact with a plain vascular membrane 

 which was at a. po, of 40 mm mercury, the^o., would fall to zero at a distance 



Figure 4. Radio-sensitivity of Ehrlich ascites tumour cells irradiated in vivo 



of 145/ \/n microns from the membrane. Since, in a five-day tumour, the 

 concentration of cells is usually about 10*/ml., n ■^ 1, so that the only cells 

 which, in the absence of stirring, would have a sensitivity appreciably above 

 the anoxic level, would be those within 145 jj, of a vascularized membrane 

 at pQ^ of 40 mm mercury. If the oxygen tension of the membrane were P mm 

 mercury, the distance would be 145 \/{Pl40) microns. The volume of a 

 five-day tumour is ~ 2 ml. If, to consider the worst case from the point of 

 view of oxygen supply, the tumour corresponded to a spherical mass of fluid 

 confined within a membrane which is maintained at 40 mm mercury, 

 the outermost 145 \x of fluid would contain 7 per cent of the fluid. The 

 expected ratio of a/a^ would therefore be that of a mixture of 7 per cent of 

 aerated cells ([Og] ~ 50 (xM/1.) and 93 per cent of anoxic cells, i.e. ~ 1-10. 

 Owing to the convoluted surface of the viscera, a somewhat higher value 

 would be expected in practice in the absence of stirring. The observed value 

 {Table 2) was 1-19. 



* Measurements by Dr. D. L. Dewey at 37 °C. 



83 



