404 BIOLOGICAL EFFECTS OF RADIATION 



resistance. For example, it has been shown recently by Henshaw and 

 Henshaw (46), using Drosophila, that during the 3 hr. after fertihzation 

 there is marked increase in resistance. This is followed during the next 

 2 hr. by a pronounced decrease to nearly the level at the time of fertiliza- 

 tion. This decrease is then followed by a subsequent rise in resistance. 

 This variation in resistance during the first 5 hr. after fertilization is not 

 shown in Mavor's curve for Drosophila (Fig. 3). Henshaw and Henshaw 

 have shown, furthermore, that the curve of resistance varies with the 

 dosage of X-rays. They have demonstrated, also, that the curve of 

 resistance to X-rays is entirely different from the curve of resistance to 

 alpha particles. When Drosophila eggs are most resistant to X-rays they 

 are most susceptible to alpha particles. This difference is apparently 

 associated with the short penetration range of the alpha particles into 

 protoplasm, and also with the movement of active cells during the forma- 

 tion of the blastula and the gastrula in the developing Drosophila egg. 



Holthusen (58) has obtained interesting results in the case of Ascaris. 

 He finds that Ascaris eggs are very resistant between fertilization and 

 the first cleavage. At the time of the first cleavage there is a period of 

 maximum sensitivity after which resistance gradually increases. 



The work of Woskressensky (94) on Axolotl and Drosophila larvae 

 presents some of the most interesting and the most accurate data which 

 are available on the relation of age to sensitivity. From the data of 

 Woskressensky it is possible to predict accurately the time of death of a 

 radiated embryo on the basis of the dosage of X-rays and the age of the 

 individual at the time of radiation. Woskressensky shows that with a 

 given dosage the sensitivity of a developing organism decreases with age 

 according to a hyperbolic function. He points out, furthermore, that 

 decrease in sensitivity is synchronous with decrease in the velocity of 

 growth. In other words, there appears to be an association between 

 growth rate and sensitivity. Such a correlation as this must not be taken 

 to mean, however, that sensitivity is dependent primarily on growth rate. 

 Although an association between growth rate and sensitivity is clearly 

 indicated, growth rate is probably only one of several factors which 

 underlie sensitivity. 



If it be true that growth rate and sensitivity are inseparably associ- 

 ated, then changes in external conditions which affect the growth rate 

 should also affect the sensitivity of a developing organism. Using 

 changes in temperature to alter the division rate of Drosophila eggs, 

 Packard (74) has tested this hypothesis and found an indication of a 

 definite correlation between division rate and sensitivity. The higher 

 the temperature at the time of exposure to X-rays, the more sensitive 

 are Drosophila eggs. In other words, as the temperature rises, the 

 division rate increases and sensitivity also increases. However, it is 

 important to note that Packard's data show that the increase in sensi- 



