450 BIOLOGICAL EFFECTS OF RADIATION 



It is evident from the foregoing studies upon amphibia that members 

 of this class among vertebrates react to radium and X-rays in a manner 

 superficially comparable with the reactions found in other animals having 

 conspicuous powers of regeneration. The healing again appears as a 

 process independent of regeneration and performed by extension of the 

 old epithelium, although the new covering may not attain normal thick- 

 ness in the absence of cell additions by multiplication or otherwise. The 

 degree of inhibition is proportional to the exposure. The extent to which 

 the effects of the irradiation are local is not fully established, although the 

 local effects seem to be more important than such general effects as have 

 been described. The complexity of the histological changes in the 

 regeneration of the adult amphibian makes it appear that any destruction 

 of reserve cells or of cells with embryonic potencies that might occur 

 would be difficult to demonstrate beyond question. In the larvae, 

 however, these changes are simpler and clues may be obtained to an 

 understanding of adult conditions. In view of what is known of the 

 histological changes during normal regeneration and those following 

 irradiation in other types of animals, there is a presumption in favor of 

 undifferentiated reserve cells as important factors, which makes this 

 perhaps the most significant clue to be followed as the case now stands. 

 While the vertebrate presents the disadvantage of this histological com- 

 plexity and also a physiological one, it has the advantage of presenting 

 tissues that are better known than those of any other animals and the 

 same may be said regarding the physiological processes of vertebrates. 

 There is also the advantage, in a case like the well-grown salamander, of 

 an animal large enough for the easy removal of parts that can be regener- 

 ated and an animal that can be maintained for long periods under 

 laboratory conditions. Since these advantages outweigh the disadvan- 

 tages, it would seem that these amphibia and perhaps other lower verte- 

 brates should be promising material for further investigations. If there 

 is any animal type that might meet the specifications for "castration" 

 against regeneration, to which reference has been made, it would seem 

 to be the members of this group. There is also the fact that what may 

 be accomplished with the vertebrates is more closely related to clinical 

 knowledge and interests that may be illumined by investigations of this 

 nature; and that existing knowledge of vertebrate physiology gives so 

 broad a foundation for study of the organismal effects of such radiations 

 upon these animals. 



PHYSIOLOGICAL REGENERATION 



What is often called "physiological" regeneration might be included 

 in the present discussion, but it would be difficult to limit the account 

 of such processes and there are many who question their inclusion under 

 the head of regeneration. The effects of X-rays and radium upon the 



