Inhibitive Action of Roentgen Rays. 193 



in nature and seemed due possibly to insufficient protection offered 

 by the thinned epithelium. Young individuals, of which we had 

 several hatched out from an egg-capsule of P. lugubris, a few 

 weeks before the experiments began, were affected like the ma- 

 ture specimens, but more quickly. 



From worms exposed from ten to fifteen times to the rays 

 pieces were cut immediately in some instances, and in some in- 

 stances a week or ten days after the last exposure. In each instance 

 the cut surfaces were closed by muscular contraction and mechan- 

 ical extension of the surface epithelium, but in no instance was 

 there subsequently seen any sign of the production of new tissue 

 at the cut surface or in a region of the body where under normal 

 conditions a new pharynx would be formed. Microscopical sec- 

 tions o'f a specimen killed within the second twenty-four hours 

 after the isolating cut was made showed no signs whatever of 

 cell-division, either direct or indirect. In control specimens mitosis 

 was most active in the tissue-forming parenchymal cells at this 

 period. In the exposed specimens the epithelium where it had 

 extended out to cover a cut surface remained a flat, thin mem- 

 brane as long as the specimen lived. In the control specimens 

 it was quickly restored to its normal columnar form. The ex- 

 posed Individuals lived for from twenty-five to thirty days from 

 the time of the first exposure. One piece of P. maculata, from 

 which the head had been remov^ed, lived for forty-one days. Re- 

 actions to light and to mechanical and chemical stimuli seemed 

 normal in all the specimens. 



From these experiments it is evident that the Roentgen rays 

 have a powerful inhibitive effect upon cell-reproduction in pla- 

 narians. It may be entirely stopped by sufficient exposure. No 

 effect was noticed in the physiological activities or in histological 

 structure of the highly differentiated tissues such as those of the 

 nervous system and the musculature. The effects of the rays do 

 not appear for some days after the first exposure. Thus there 

 is a slight production of regenerative material at a cut surface 

 in a specimen sectioned before exposure to the rays. The sub- 

 sequent differentiation of an imperfect eye in one specimen in- 

 dicates that the rays have effect not so much upon tissue differ- 



