414 BIOLOGICAL EFFECTS OF RADIATION 



evidence for division in the large cells of both ectoderm and endoderm 

 and raised the question whether the division of interstitial cells so com- 

 monly observed during regeneration might not be related primarily to 

 the formation of new cnidoblasts and their nematocysts, which the 

 regenerate produced in large numbers when tentacles and hypostome 

 were being restored. Hadzi (20, 21) concluded that buds arose from 

 interstitial cells, and Schulze noted (53) that all tissue elements of the 

 hydra arose from such cells. Gelei (18), however, found mitoses fre- 

 quently in all cell types of Hydra grisea, except cnidoblasts and the cells 

 of the tentacles, and concluded that the whole organism was not far from 

 an embryonic state. Kanajew (29, 30) regarded the regeneration in 

 Pelmatohyd.ra oligactis as a result of the transformation of interstitial 

 cells into other cell types, since he found mitoses rare in the differentiated 

 cells and abundant in the undifferentiated or interstitial cells. He 

 described the latter as migrating through the supporting lamella and then 

 differentiating into the new endoderm cells. In a later study (31) his 

 earlier observations were not substantiated, and he concluded that the 

 interstitial cells played an insignificant part in regeneration and budding, 

 since regenerating parts and buds were formed principally at the expense 

 of the neighboring differentiated cells, as shown by histological study and 

 by the transplantation of parts that had been vitally stained. The 

 observations of Strelin (61) upon Pelmatohydra oligactis further support 

 the concept of an embryonic role for the interstitial cells; but this inter- 

 pretation is not supported by the observations of McConnell (43) who 

 described abundant mitoses in the epitheliomuscular cells of P. oligactis 

 and hence concluded "there is no evidence that the indifferent interstitial 

 cells of the ectoderm or endoderm are elaborated into the large epithelio- 

 muscular cells," and further, in a second paper (44), that the secretory 

 cells of the endoderm also reproduced by mitosis. There seems no 

 reason to consider the possibility of amitosis in hydras, since only mitosis 

 has been recorded by recent investigators. 



With such contrasting views one despairs of a solution of the problem 

 by the ordinary methods of histological study, although the presumption 

 favors the conclusions of investigators such as Kanajew, who has changed 

 his opinion, or McConnell, whose microphotographs demonstrate the 

 existence of mitoses which he describes as frequent, rather than the 

 opposite conclusions of Strelin (61) and others who may have overlooked 

 mitoses in their material. There is also the possibility that species of 

 hydras differ in this particular, or that the members of a species exhibit 

 seasonal differences. A new mode of observation or experimentation 

 might yield facts that would be more convincing than any available by 

 older methods. If, for example, one could destroy all the interstitial 

 cells without too great injury to the individual, it would be possible to 

 determine the potencies of the cells that remained. It appears that such 



