REGENERATION 



417 



functions apparently dependent upon these cells had been lost, yet, m 

 spite of these losses, organisms in which the power to live indefinitely 

 persisted, just as a mammal sterilized by irradiation may live out its 

 usual span of existence. This might be impossible with the hydra, 

 because some necessary function of the organism, such as the formation 

 of nematocysts, might be lost with the destruction of the interstitial 

 cells, and further because the normal span of life could not be determined. 

 Many investigators have ascribed a similar embryonic role to the 

 interstitial cells in marine hydrozoa ever since the investigations of 



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Fig. 2. — Effects of beta rays of radium upon regeneration in Tuhularia crocea. Left — 

 curves showing acceleration of hydranth regeneration due to seven hours' exposure; 

 ordinates indicate percentage of development; abscissas, hours from time of cutting; broken 

 line represents controls; continuous line, irradiates; bracket indicates period of exposure. 

 Right — curves showing retardation of hydranth regeneration from a 27-hr. exposure; 

 details as in last. (From Congdon, 11.) 



Weismann upon hydroids. Others have disputed this doctrine; but the 

 histology of these hydrozoa has not been so exhaustively studied as that 

 of hydra. Congdon (11) exposed pieces of the stems of Tuhularia crocea, 

 from which the hydranths had just been removed, to beta radiations from 

 300 mg. of impure radium for periods up to three days in length. He 

 found that the shorter exposures accelerated and that the longer ones 

 retarded the regeneration. The degree of retardation increased slowly 

 with lengthening exposure; but the degree of retardation relative to the 

 length of exposure decreased with lengthening exposure. In order to 

 express the amount of regeneration numerically, the whole process of 

 restoring a hydranth was divided arbitrarily into eight stages as observed 

 during the three or four days involved, so that the stage of regeneration of 

 each piece could be calculated for exposed and for control sets. The 

 results are indicated by Fig. 2 which shows that acceleration and retarda- 

 tion result from different types of exposure. By means of correlation 

 tables, appearing as his Figs. 7 and 8, Congdon showed the conditions 

 determining whether acceleration or retardation would occur. Upon 

 the whole it appeared that the exposures producing acceleration were the 



