428 BIOLOGICAL EFFECTS OF RADIATION 



increased (Fig. 8). There was no evidence of a stimulating effect in the 

 action of the rays. The highly differentiated cells of the gut, sense 

 organs, nervous system, and musculature remained unchanged until 

 affected by the final deformation and degeneration of the piece. 



It appears from these studies by various investigators that the 

 inhibition of regeneration in planarians by X-rays and radium involves 

 destruction, or at least injury, of the cells that exhibit embryonic potencies 

 in normal regeneration. The changes observed in the chromatin and in 

 the mitoses, as in other instances of irradiation, suggest that the effect is 

 directly upon these cells and not an indirect effect resulting from a 

 changed physiological state of the organism as a whole. In other words, 

 cellular changes in this instance seem to precede the organismal changes 

 appearing as the failure of regeneration and the degeneration that 

 follows. 



In a brief and presumably a preliminary statement, Meserve and 

 Kenney (41) have reported a study of the effects of X-rays upon different 

 sized populations of Planaria dorotocephala totaling over 800 individuals. 

 They state that the primary effects, which appear during the first 2 weeks, 

 are largely of a nonspecific nature, since the head forms developed 

 resemble those induced by various depressants except for one type of head 

 formation. After any one exposure of 4, 8, or 12 skin units, the range of 

 distribution of types regenerated increases as crowding increases. A 

 subsequent disappearance of tissue differentiated during the first 2 weeks 

 seems to be an effect of X-rays on planarian tissue that gains expression 

 after regeneration has reached its limits and that is first apparent in the 

 region having highest rates of metabolism. 



Secondary effects were observed, in all of the X-rayed forms receiving 

 4, 8, and 12 skin units, as cytolytic changes, which first became promi- 

 nent 34 days after exposure and resulted in death. The authors state, 

 although there is nothing said regarding a study of the cells by means 

 of sections or otherwise, that their results agree with those of Bardeen 

 and Baetjer (3), "who conclude that X-rays affect cell division and cell 

 differentiation and that the effects are probably confined to these two"; 

 that "cell differentiation is not as much affected as cell division"; and 

 that "the effect upon cell division is not direct." No reference is made 

 to the work of Weigand (67) or to the very limited observations which 

 Bardeen and Baetjer made upon the cellular changes, as noted on 

 page 420. 



It is further stated by Meserve and Kenney, that "while both the 

 more immediate effects and the delayed effects of X-rays may be specific 

 upon the protoplasm, it does not necessarily follow that, because head 

 frequency is affected by X-rays, the factors which control head frequency 

 are specific and directly related to the activity of special formative cells. 

 The formative cell theory of Curtis does not recognize the fact that the 



