EMBRYONIC DEVELOPMENT 



403 



were younger embryos. Embryos which had been incubated 25 hr., for 

 example, were more resistant to X-rays than embryos which had been 

 incubated 6 days. Such a result as this raises the question as to whether 

 the tissues of the older embryos are actually less resistant than the tissues 

 of the younger embryos, or whether, owing to the more complex organiza- 

 tion of the older embryos, radiation produces physiological disturbances 

 which are either absent or less pronounced in the younger embryos. 



M.A.M 

 cr,4200 



30 60 90 120 150 180 210 240 270 300 



<Eggx- Lcrrvci ■>-<- Pupa 5>-«s---Imoigo- — 



Age When Rayed in Hours 

 Fig. 3. — The variation in susceptibility to X-rays during the development of Drosophila. 

 The ordinate represents the X-ray dose expressed in milliampere minutes, which was 

 required to kill 50 per cent of the indi\aduals within five days after the radiation. The 

 abscissa represents the age of the individuals in hours at the time of the radiation. {From 

 Mavor, 66.) 



Tissue-culture studies of Strangeways and Fell indicate that the latter 

 possibility is undoubtedly true, for, as they have shown, the cells, qua 

 cells, of a 6-day chick are no more susceptible to the action of X-rays 

 than those of a 25-hr. chick. Strangeways and Fell beUeve that the death 

 of radiated 6-day chick embryos is due, in part at least, to the inter- 

 ruption of gaseous exchange in the tissues of the chicks, such as might 

 result from the clotting of blood in the vessels shortly after radiation. 

 The developing chick embryo, therefore, becomes an exception to the 

 generalization of Bergonie and Tribondeau, because we are dealing in the 

 case of the chick with an indirect action of X-rays instead of with 

 the direct action of the radiation upon the cells and tissues. 



The greatest amount of evidence at hand, therefore, indicates that 

 with a few exceptions, such as the chick, there is a gradual increase in 

 resistance to X-rays and radium as development of an organism progresses. 

 Nevertheless, in some animals fluctuations occur in the upward curve of 



