398 BIOLOGICAL EFFECTS OF RADIATION 



of early developmental arrests in the neural tube which are caused by the 

 radiation. 



The fact that the systems of the embryonic body, both in amphibians 

 and in birds, which are most severely affected by radiation are the nervous 

 and the vascular systems indicates a possible correlation between meta- 

 bolic rate and radiosensitivity. The nervous and the vascular systems 

 in early development are the most rapidly growing systems of the body. 

 The regions where these are developing are undoubtedly the regions of 

 highest metabolic rate. That these regions should be the most sensitive 

 to radiation is very indicative of a correlation between metabolic rate 

 and sensitivity. 



Strangeways and Fell (88) have tested the possibility of a correlation 

 between metabolic rate and sensitivity by using the method of tissue 

 culture. Chick embryos after 6 days of incubation were exposed to 

 X-rays and then placed for 5 hr. in a refrigerator at 0°C. At the end of 

 the 5 hr. at 0°C. tissue fragments of the heart, the sclerotic, the choroid, 

 the intestine, and the skin were transplanted and grown in vitro. In the 

 case of these cold-treated embryos fairly good growth was obtained 

 from all of the tissues cultivated. On the other hand, in tissue cultures 

 made from chick embryos which were incubated for 5 hr. after the radia- 

 tion no growth at all, or at best scanty growth, took place in any culture. 

 From these results it appears, therefore, that the degenerative changes 

 produced in the tissues by radiation are greatly delayed, if not com- 

 pletely arrested, by maintaining the embryos immediately after the 

 radiation at a low temperature and thereby lowering the metabolic rate. 



Concerning the relationship between sensitivity and metabolic rate 

 in amphibian development, Ancel and Vintemberger (1, 6) have come to 

 conclusions similar to those of Strangeways and Fell. Ancel and Vintem- 

 berger have found in their work with amphibian eggs that the extent of 

 injury is a function of the work which normally is done after the radiation. 

 In other words, the activity of a cell after the exposure determines to a 

 great extent the degree to which the cell will be injured. If cellular 

 activity after radiation is great, then the cells which are suffering from 

 the effects of the radiation are unable normally to perform these activities 

 and as a result abnormalities occur. On the other hand, if cellular activ- 

 ity after radiation is slight, then the cells may be able to recover from 

 the effects of the radiation and no serious disturbances may result. 

 Packard (74) has arrived at a similar conclusion in his studies on Dro- 

 sophila eggs, which will be discussed later. 



Another question with regard to the degenerative effects of X-rays 

 and radium on embryonic tissues is whether these effects are due to a 

 direct action of the radiation on the cells and tissues, or whether they are 

 due to an indirect action resulting from the accumulation of toxic sub- 

 stances in the body fluids. In answer to this question, with regard to 



