REGENERA TION 44 1 



Butler (7) studied the effects of X-rays upon regeneration of the 

 fore Umb in larvae of Amhly stoma pundatum, using a Coolidge medium 

 focus tube operated at 60 kv., 6 ma., 25 em. from target to specimen, 

 and without a filter. The exposures are not stated in roentgens, but the 

 foregoing factors were constant throughout. The duration of exposure 

 was varied in different experiments, but in each case a daily exposure, 

 of 2, 3, or 5 min., respectively, was made throughout periods of from 21 

 to 25 days according to the experiment. The question of the exposure 

 necessary to prevent regeneration remained to be thoroughly investi- 

 gated. It appeared that the exposures used in some of the earlier experi- 

 ments were much longer than was actually necessary, since it was found 

 in later experiments that daily exposures of 1 min. each were as effective 

 as the daily exposures of longer duration. The position of the larvae 

 during exposure is not stated. 



In a representative case, larvae 15 mm. in length with fore limbs in 

 early four-digit stage were irradiated in two lots 2 and 3 min. daily for 

 a period of 21 days, beginning the day following amputation of one fore 

 limb on each irradiate. In none of these irradiates was there any sign 

 of regeneration of the amputated fore Umb. In the controls there was 

 rapid and normal regeneration. There was no significant difference 

 between the larvae irradiated 2 min. and those given the 3-min. exposure. 

 In another experiment, larvae with fore limbs in early two-digit stage 

 were irradiated 5 min. daily for 25 days following amputation. A 

 similar result was obtained by amputation of the limb buds before appear- 

 ance of the digits. 



Comparable results were obtained by Butler in 23 experiments of a 

 like nature. He found that the amputated limb failed to regenerate and 

 also that there was usually a considerable resorption, so that the non- 

 regenerating limb stump was smaller several weeks after amputation 

 than the portion remaining attached to the body the few days after the 

 operation. The effect of the rays upon digit formation Butler regards as 

 significant. He found when a normal and uncut limb was about to enter 

 the two-digit condition at the time radiation began, that the first two 

 digits would be formed in a manner apparently normal, but no third digit 

 would ever develop. In like manner, if the third digit was just appearing 

 at the time radiation began, the third digit would be completely formed, 

 but no fourth digit would ever develop. These results suggested 

 that the X-rays acted upon differentiating tissue at a certain critical 

 period in the process during which there was a heightened sensitivity 

 to these radiations, and that if the differentiation had passed this period 

 before the exposures began, it proceeded despite the irradiation. From 

 other experiments in progress but not included in the paper under discus- 

 sion, Butler thought that a similar critical period might exist in the 

 process of regeneration. 



