REGENERATION 421 



at the time of this work, was also recorded. The fact which was then 

 being recognized that the effects of these radiations upon organisms were 

 due "primarily to their action upon cells capable of reproductive activity" 

 and which was soon to be formulated as the law of Bergonie and Tri- 

 bondeau, was thus supported by the work of these investigators. After 

 so comprehensive, although brief, a survey of the effects in planarians 

 it is surprising that other studies upon the effects of such radiations upon 

 regeneration did not follow immediately. The paper by Schaper (51) 

 recording observations on the effects of radium emanation upon embry- 

 onic and regenerative processes contains nothing regarding planarians, 

 except a very brief account of a similar check upon regeneration by 

 exposure of Planaria luguhris to radium. In none of this early work 

 was it possible to determine exactly what exposures were being given, 

 as can be done in current experimentation. 



So far as the writer of the present article is aware, the next studies 

 upon the effects of X-rays in the regeneration of planarians were those 

 by himself and his students, as variously reported before the American 

 Society of Zoologists (15) and the work of Weigand (67). These investi- 

 gations were undertaken as a renewal of earlier studies upon the histo- 

 logical changes of regeneration of planarians (12) in which the importance 

 of a reserve stock of "formative cells" had been indicated. It was 

 thought that the abundance of these cells in different species might be 

 correlated with the contrasting powers of regeneration that had long been 

 known to occur in planarians, and this appears to be the case in Planaria 

 maculata as compared with Procotyla fluviatilis (14). The technique of 

 irradiation gave promise in the attack upon this problem and the results 

 justified the expectation (Fig. 4). With exposures ranging from 2500 

 to 3500 r planarians such as P. maculata and P. agilis, which have great 

 powers of regeneration and many formative cells, were rendered incapable 

 of regeneration, although some of the worms remained alive for 30 days 

 or more before beginning to show abnormalities that were the forerunners 

 of death. Somewhat lighter exposures produced individuals that regener- 

 ated slowly or imperfectly and with a minimum of new tissue at the cut 

 surfaces. In some instances such individuals recovered from the effects 

 of the irradiation and regenerated normally when cut 60 or 70 days after 

 the date of exposure. When irradiated worms were sectioned, it was 

 found that the formative cells were proportionately reduced in numbers. 

 In the heaviest exposures these cells were so far eliminated that there 

 seemed to be none remaining in many of the specimens (Fig. 5). The 

 mesodermal region was thus changed from its characteristic appearance 

 in Planaria agilis to that seen in the postpharyngeal region of Procotyla 

 fluviatilis (14) which is incapable of regeneration. In experiments 

 with radium, in which an exposure about equivalent to that with the 

 X-rays was used, regeneration was inhibited in a similar manner. 



