BIOLOGICAL EFFECTS OF RADIATIONS 707 



also been found that embryos become more resistant to radiation 

 as they develop. Exceptions to this conclusion that the embryos 

 become more resistant as they develop are stages of extreme sus- 

 ceptibility, such as gastrulation which involves many critical proc- 

 esses not experienced later, and stages of unusually high resistance, 

 such as occur in some insects during early cleavage when the un- 

 differentiated embryo has remarkable regulative powers. Roentgen 

 radiation has been employed in upsetting the normal differentiation 

 of embryos, and the results have shed some light on inter-relation- 

 ships of cells at different stages in development. It has been found 

 that injury to young embryos as a result of irradiation tends to be 

 widespread, whereas radiation injury in older ones is more localized. 

 Evidence indicates that differential susceptibility of tissues of the 

 adult is not the same as in embryos. For example, the adult nerve 

 and bone tissues are apparently resistant, whereas in growing em- 

 bryos these tissues are susceptible. 



One of the interesting and more practical types of experiment 

 with roentgen radiation has been that of determining the effects on 

 regeneration. It has been found that the effects are localized to 

 regions of a body which have been exposed, and regeneration of 

 amputated limbs can be inhibited by proper exposures even though 

 the wound may heal over. This may be considered as evidence, 

 which is in accord with other embryological experiments, that cellu- 

 lar differentiation can be effected without necessarily destroying the 

 ability of cells to increase in number. 



The radiologist (x-ray physician) and the x-ray manufacturers, 

 as well as the biologist, have long recognized the need of more 

 information concerning the fundamental action of roentgen radia- 

 tion upon cells. Radiological organizations, some x-ray manufac- 

 urers, and such scientific groups as the Committee on Radiation of 

 the National Research Council and the American Association for the 

 Advancement of Science have aided such investigations. 



The Fundamental Action of Roentgen Radiation 



The photon of roentgen radiation is capable of removing an extra- 

 nuclear electron from an atom leaving it temporarily charged as an 

 ion. The electron removed by the photon is called a pJiotoelectron 

 and is capable of removing other electrons from lower energy levels. 

 Thus the path of the photon (until all of the energy is absorbed) 

 is marked, momentarily, by a trail of ions. In such a manner mole- 



