418 BIOLOGICAL EFFECTS OF RADIATION 



shorter, while the amount of retardation varied directly with the length 

 of the exposure. He concluded further that there was "much to indicate 

 that, if time be given for the cut pieces to partially regenerate before 

 exposure, the effect of the beta radiation is decreased"; and that "differ- 

 ing amounts of regeneration before exposure played only a minor part in 

 the various retardations and accelerations." However, "the retarding 

 effect does not increase so rapidly as the length of exposure, though within 

 the limits of these exposures, it does continue to increase. In other words, 

 the sensitiveness of the hydroid decreases as the length of exposure 

 increases." According to Congdon, this might be "simply an expression 

 of Weber's law, that beyond a certain maximum of intensity any stimulus 

 has a decreasing power of stimulation. Or, it may be that the early 

 stages of regeneration are more sensitive to exposures for the same reasons 

 that the embryo is more sensitive than the adult." 



Curtis and Ritter (15), in a study of Tuhularia crocea demonstrated 

 that the power of regeneration could be completely inhibited by X-rays. 

 The data relating to these exposures are: 65,000 v., 5 ma., thin card- 

 board filter, emission 31.4 r per min. Small masses from a colony, with 

 basal entanglement of the stems intact, were spread fanwise to form an 

 approximately single layer in a Petri dish containing a measured amount 

 of water that was estimated to be not more than 8 mm. deep in any part 

 of the dish. There was necessarily considerable difference in the depth of 

 water that screened any one part of a stem or hydranth from the rays. 

 With exposures in different experiments ranging from 30 to 120 min. 

 it was found that 90 min. was sufficient for inhibition of hydranth regener- 

 ation in almost every instance. Thus, with a 90-min. exposure the 

 regeneration of hydranths, which was normally accomplished within 

 48 hr., was completely inhibited, although the stems remained alive for 

 upward of 20 days, which was as long as it was possible to maintain 

 normal stems or colonies under the artificial conditions of the experi- 

 ments. The irradiated stems could produce outgrowths resembling 

 hydrorhizae, apparently by rearrangement of existing material, but never 

 hydranths when the exposure was sufficient. Externally the stems were 

 normal to all appearances when compared with the living controls in any 

 experiment, and internally the normal circulation within the enteron 

 was maintained. This latter feature indicated that the physiological 

 state of the endoderm cells could not have been much disturbed by the 

 radiation. Such nonregenerates could not live indefinitely without 

 hydranths. They did live as long as normal stems and portions of the 

 colonies remained alive under similar conditions. During such a 3-week 

 period a control stem might have its hydranth removed 3 or 4 times 

 with subsequent regenerations, while the X-rayed stems after similar 

 removals of oral portions showed only a series of healings. In this case 

 the irradiated piece seems to meet the specifications for continued exist- 



