1324 



BIOLOGICAL EFFECTS OF RADIATION 



necessary for a division of the nuclei is according to Huettner (49) 10 min. 

 at 23°C. Packard (75) places the time at 1 hr. at 13°C. and 15 min. at 

 28°C. The numbers of nuclei in the eggs would increase in geometrical 

 ratio with time. If the cause of death in irradiation is the production of 

 dominant lethal mutations, the susceptibility of the egg should change 

 from the time it is in the single-nucleus stage, when theoretically it should 

 be twice as susceptible as the sperm, to double the susceptibility at each 



8 920 17 840 26 760 



REOUCEO SCALE - 



35 680 



X-RAYS (eSu/SEC/CM^; 

 AMOUNT OF IRRADIATION 



FiQ. 4.— Survival curves for Drosophila eggs (at 1 to 3 hr. from laying) , chromatin diploid, 



and sperm, chromatin haploid. 



rhythm of cell division provided each nucleus is essential to further devel- 

 opment. According to Packard's work, the Drosophila eggs do increase 

 in sensitivity until they are most sensitive at 2 hr. from laying, when 

 they become less sensitive. This becoming less sensitive seems also to 

 have a meaning in the course of development. Patterson (79, 80) and 

 Gowen (23) showed that the anlage of the adult eye of Drosophila was in 

 the one-celled stage not longer than 12 hr. after the egg was laid and 

 very possibly less time than that. If the vital organs differentiate even 

 more rapidly so that they become two-celled early in development, 

 requiring that both be destroyed to cause the egg's death, then a multiple- 

 absorption curve will be necessary to account for the observed deaths. 

 This increase gives the appearance of increased resistance of the cell to 

 X-rays after such a critical period is passed. The data are not accurate 



