402 BIOLOGICAL EFFECTS OF RADIATION 



amount of available evidence points to the period of gastrulation and 

 body-axis formation as the time when abnormalities in radiated embryos 

 most frequently occur. On the other hand, embryos which survive this 

 period of relative instability, or which are not radiated till gastrulation is 

 past and the body axis is established, frequently exhibit serious abnor- 

 malities in the nervous and vascular systems and, in some cases, in the 

 urinogenital system. 



The other aspect of the problem of sensitivity, which was referred to 

 above, deals with the relation between sensitivity and age. This problem 

 is concerned not with the immediate or subsequent appearance of 

 abnormalities ; it is concerned rather with a determination of the amount 

 of radiation which an egg or an embryo can withstand at a particular 

 time and still remain viable, and with the question of a change in resist- 

 ance with relation to age. 



Bergonie and Tribondeau (21) have made a generaUzation with regard 

 to radiosensitivity which has often been referred to as the Bergonie- 

 Tribondeau law. This law holds that the sensitivity of cells varies 

 directly with their reproductive capacity and inversely with their degree 

 of differentiation. According to the Bergonie-Tribondeau law, therefore, 

 sensitivity should decrease during embryonic development ; older embryos 

 should be more resistant than younger embryos. A review of the work in 

 which any quantitative data at all are presented indicates that in most 

 cases this generalization holds true. Bardeen (20), for example, observed 

 that young stages in amphibian development were much more sensitive 

 than older stages. The increase in resistance in the amphibian egg 

 during development has been studied more recently by Woskressensky 

 (94) who has collected important quantitative data. Perthes (79) in his 

 early work on Ascaris reported that later stages in development were 

 less sensitive than earlier stages. Holthusen (58) found in Ascaris eggs 

 that maximum sensitivity occurred at the time of the first cleavage. 

 After the first cleavage the sensitivity of the developing Ascaris gradually 

 declined. In the development of the tobacco beetle, Runner (84) found 

 that sensitivity decreased with age. Likewise, Mavor (66) has shown 

 in Drosophila that the egg and larval stages are relatively very sensitive 

 and that sensitivity gradually decreases during pupation. At the end 

 of pupation an individual is approximately 35 times as resistant to X-rays 

 as at the beginning of pupation, and nearly 100 times as resistant as 

 at the time when the egg was laid. Figure 3, taken from Mavor's work, 

 shows graphically the increasing resistance of the Drosophila egg during 

 development from the egg to the imago. 



In the development of a few organisms the generalization of Bergonie 

 and Tribondeau apparently does not hold true. In the work of Strange- 

 ways and Fell (88) on the developing chick it has been shown that older 

 chick embryos were more susceptible to destructive X-ray effects than 



