420 BIOLOGICAL EFFECTS OF RADIATION 



contraction and subsequent extension of the old epithelium as in controls, 

 but the epithelium remained thin and did not become columnar as is 

 normally the case. There was a slight production of new tissue at the 

 cut surfaces in some of the worms cut just before or soon after the first 

 exposures and hence without allowance for a latent period in the action 

 of the rays. There was no new tissue at these surfaces, or in the region 

 where a pharynx would have been formed in a normal regenerate, when 

 the specimens were cut well after a sufficient exposure. Microscopic 

 examination of a few specimens, taken at varying periods after the last 



Fig. 3. — Source of formative cells in planarians according to the conflicting theories of 

 these cells as a persistent embryonic stock and as arising by dedifferentiation. {From 

 Curtis and Schulze, 14.) 



exposure in one of the experiments, showed no marked alteration in the 

 specialized cells of the muscular, nervous, and digestive systems ; but the 

 testes showed none of the mitotic divisions that were abundant in testes 

 of controls. A single specimen from another experiment killed within 

 the second 24 hr. after being cut for regeneration showed no signs of cell 

 division, although controls showed abundant mitoses in the "tissue- 

 forming parenchymal cells" at this period. After irradiation the physio- 

 logical activities of whole worms were apparently normal as shown by 

 "normal reactions to light, to mechanical, and to chemical (food) stimuli." 

 The authors did not describe the external changes just preceding death, 

 except to say that death resulted from a "degenerative process which 

 began in the region of the head and extended slowly back." A few 

 recently hatched individuals, which they observed, were "affected like 

 the mature specimens but more quickly." 



From these observations Bardeen and Baetjer concluded that "the 

 power of regeneration may be completely destroyed by exposing planar- 

 ians to the action of the roentgen rays " ; that these rays "have a powerful 

 inhibitive effect upon cell reproduction in planarians," and that this cell 

 division "may be entirely stopped by sufficient exposure." Evidence 

 for a latent period in the action of the rays, which was known to clinicians 



