REGENERATION 439 



general, with one or both Umbs screened from the rays to the level of the 

 thighs and the remainder of the specimen exposed; or local, with the 

 limbs exposed and the remainder screened. The animals were anes- 

 thetized with ether before irradiation and the exposures made in a dorso- 

 ventral direction. In most cases the limbs were amputated 1 to 2 hr. 

 after the irradiation, except in the experiments designed to test the 

 persistence of the effect. He found that a sufficient general exposure 

 killed all the irradiates in any experiment, but the time of death depended 

 upon the age of the individual as well as the strength of exposure. Thus, 

 year-old specimens exposed to 600 r died 19 to 27 days later, while nine- 

 months-old specimens exposed to 330 r died at 25 to 30 days. A similar 

 dual relationship was observed in the inhibition of regeneration. The 

 yearlings exposed to 330 r showed complete inhibition of regeneration 

 when their limbs were amputated, while nine-months old individuals 

 exposed to only 88 r showed a similar inhibition. With lesser irradiations 

 the period of survival varied inversely with the exposure and the regenera- 

 tion was less affected until at an exposure of 44 r there was complete 

 survival and subnormal regeneration, and at 22 r both survival and 

 normal regeneration. Local exposures of limbs and fins, instead of 

 general ones, were used exclusively after it had been found that the 

 results in local and general exposures were similar, because with local 

 exposures the experiments could be made without the complication of 

 lethal effects due to the irradiation. The only disadvantage was that 

 regeneration proceeded more slowly following local exposure of a limb 

 than following a general exposure. 



Despite the effects observed in animals that had received general 

 exposures, Litschko concluded that the inhibition was due to local action 

 alone. The failure of regeneration of the limbs after certain local expo- 

 sures (88 r for nine months-old specimens) seemed to result from the 

 existence in the limb of certain conditions under which the power of 

 regeneration could not express itself and not from a complete destruction 

 of this potency. Histological study showed that inhibition in the out- 

 growing stump of a regenerating limb was correlated with the formation 

 of a mass of connective tissue at the distal end into which it seemed that 

 the muscle and other underlying tissues could not extend themselves as 

 normally. Removal of the whole of this terminal mass of connective 

 tissue, by a second amputation made before degeneration of the under- 

 lying tissues had begun, resulted in a complete regeneration, while lesser 

 removals resulted in lesser degrees of regeneration. It thus appeared 

 that the regenerative potency had not been lost but had been inhibited 

 by the action of the rays upon the connective tissue. This action he 

 regarded as a stimulation and found additional support for such an 

 interpretation in the superregeneration observed where a portion of a 

 locally irradiated region had been cut from the dorsal fin (Fig. 15). In 



