REG EN ERA TION 451 



growth of hair has long been known and the effects upon certain glands 

 is another indication that these radiations may inhibit normal restorative 

 processes in a way that can be compared with their effect upon the type 

 of regeneration under consideration. To review the literature bearing 

 upon such a comparison would carry one far into the field of clinical 

 medicine. As an introduction to the more general biological study of 

 these "physiological" processes, as affected by X-rays and radium, the 

 recent paper by Hirsch (25) regarding secretion may be cited. 



DISCUSSION 



The foregoing resume of investigations in this field shows that radium 

 and X-rays have a similar inhibitive effect upon regeneration in animals 

 as representative and as widely separated as coelenterates, flatworms, 

 segmented worms, and chordates. Externally, the same kind of changes 

 appears in each of these cases. Healing occurs in the normal manner by 

 an extension of the old epithelium, but the subsequent thickening and 

 restoration of the original state of these cells that occur in normal regen- 

 eration do not follow. The regenerative processes by which the missing 

 parts become restored are retarded or completely inhibited according to 

 the degree of exposure to the rays. Internally, it seems that the clue 

 to the changes observed lies in the effect of the irradiation upon whatever 

 cells are active in producing the new tissues by division, migration, and 

 differentiation. One pictures the normal regeneration as involving in the 

 first instance histological changes incident to the mutilation of a given 

 region, and also organismal changes or those which involve the individual 

 as a whole and are not clearly recognizable in certain of its cells. These 

 latter processes doubtless bring about other histological changes as the 

 regeneration proceeds. While the organismal and the histological 

 factors must be regarded as so inextricably mingled that they are merely 

 different aspects of a unified series of events, it is convenient to consider 

 them separately wherever definite changes of either sort become apparent. 

 The histological aspects are more easily recognized insofar as they consist 

 of morphological and regional changes in cells. These include: the 

 nature and origin of the cells concerned in regeneration; their growth, 

 division, movement, and the like, as activated, perhaps, by organ- 

 ismal factors; and their differentiation under the influence of organizing 

 centers. Thus by the interplay of these general and specific factors 

 the regeneration is consummated. Whether one puts the organism or 

 its cells first in such a case depends to some extent upon one's concept of 

 the individual in relation to its cells as well as the data from observation 

 and experiment. 



In some instances, such as the planarian, the effect of the irradiation 

 upon specific cells appears to be a direct effect, since it is recognizable 

 soon after the exposure. In a case of this sort it is easy to think in terms 



