452 BIOLOGICAL EFFECTS OF RADIATION 



of the differential susceptibility of cells, which has been so often demon- 

 strated in accordance with the law of Bergonie and Tribondeau, as 

 responsible for the primary effect of the irradiation; and of later effects 

 as an outcome of such primary effects upon these cells. For example, 

 the degenerative changes that appear ultimately in a planarian may 

 result from toxic products formed by destruction of the considerable 

 amount of protoplasm composing the regenerative cells. In a case where 

 no effect is apparent until a later period, the delay may be due to changes 

 within certain cells not becoming recognizable soon after the irradiation 

 or to an organismal effect that does not at first become evident as changes 

 in any of the cells. In the absence of specific evidence it is perhaps an 

 academic question whether the changes observed in a given cell originated 

 as "direct hits" within that cell or have resulted, for example, from 

 changes induced in body fluids and so affecting the cell indirectly. Even 

 the seemingly direct action upon cells in tissue cultures is open to these 

 two theoretical explanations. What is needed is specific evidence if it 

 can be obtained. 



Passing over this question of direct or indirect effects and beginning 

 with recognizable cell changes, we have as the obvious explanation of 

 what can be observed the differing susceptibility of different types of 

 cells to these radiations. In a case, such as the planarian, it is known that 

 the so-called regenerative or formative cells, whether they have originated 

 by dedifferentiation or by multiplication from an embryonic stock, 

 differentiate into the various cell types and form the missing parts during 

 regeneration and hence may be said to have embryonic potencies. These 

 are the cells most sensitive to the irradiation in question, and with their 

 reduction in numbers or complete destruction regeneration is corre- 

 spondingly affected. Again, in the hydra regeneration is checked and 

 the interstitial cells, which have been described by many investigators 

 as having embryonic potencies and as being most important in regenera- 

 tion, are the cells conspicuously affected in the first instance. In oli- 

 gochaetes it is the neoblasts, in Clavelina it is the cells described as active 

 during regeneration, in the limb of Ambly stoma larvae the cells of the 

 blastema, while in the tadpoles of Pelobates the different types of cells 

 exhibit differing susceptibilities which account for the gross differences 

 between controls and irradiates. One could hardly expect a greater 

 coincidence of the internal effects and of the external effects of these 

 radiations in checking regeneration, whether in the tentacle of a hydra 

 or the tail of a tadpole. Moreover, the histological changes indicated 

 are in agreement with the widely demonstrated effects of radium and 

 X-rays upon less differentiated cells, and upon cells with embryonic 

 potencies. The consistency of all these observations is such that one 

 feels justified in considering the potencies of specific types of cells as one 

 of the most important factors in the regenerative process. While this 



