REGENERATION 419 



ence during as long a period as a normal individual, but the situation is 

 complicated by existence under laboratory conditions and by the fact 

 that this hydroid appears in shallow waters at Woods Hole, Mass., about 

 May 15 and dies in these habitats during July, although it may still be 

 found in deeper water. The histological changes following the irradia- 

 tion remain to be studied. It is known, however, that this species 

 possesses many interstitial cells. 



It appears, then, with coelenterates that lighter exposures to X-rays 

 and to the beta rays of radium result in some acceleration of the rate, 

 while heavier exposures result in a retardation and ultimately a destruc- 

 tion of the po'^-er of regeneration. In hydras both the acceleration and 

 the retardation have been correlated with the effects of the radiations 

 upon the interstitial cells which many investigators have thought capable 

 of producing all the other cell types, and therefore possessing embryonic 

 potentialities. The destruction of embryonic cells might be expected 

 by these radiations in accordance with the law of Bergonie and Tri- 

 bondeau, and such destruction would account for the loss of regenerative 

 power. Irradiation furnishes a technique by which the problem of 

 differentiation in the regenerating organism may be attacked in a more 

 advantageous manner than by any mechanical operation or by any 

 technique of staining unless it be the use of vital dyes. 



TURBELLARIA 



Among platyhelminthes, the fresh-water planarians have been most 

 extensively studied for regeneration. In recent years investigators of 

 the histological changes have commonly agreed that the so-called 

 "formative" or "regeneration" cells of the mesodermal region are the 

 source from which new organs and tissues arise during regeneration. 

 There is, however, disagreement regarding the origin of these cells (Fig. 3). 

 Investigators such as Vandel (66) and Steinmann (55 and 56) maintain 

 that formative cells originate by dediiferentiation, while others such as 

 Curtis and Schulze (14) regard the formative cells as a persistent embry- 

 onic stock and mamtain that the differing powers of regeneration in 

 certain genera of planarians may be related to the relative abundance of 

 these cells. Despite much investigation of its role in regeneration, the 

 parenchyma of planarians, as Bartsch (4, page 199) puts it, is "ein sehr 

 beliebtes Streitobjekt aller Planarienforscher." The technique of 

 irradiation has been applied to this problem. 



In a brief but comprehensive paper, Bardeen and Baetjer (3) described 

 the effects of X-rays upon regeneration in Planaria maculata and P. 

 luguhris. The worms were exposed to "medium soft" X-rays from 

 10 to 20 min. each day for a period of from 12 to 18 days, after which 

 they remained alive 20 to 30 days from the beginning of the various experi- 

 ments. In the exposed worms, healing occurred by an initial muscular 



