436 BIOLOGICAL EFFECTS OF RADIATION 



The effect upon early prophase stages as estabUshed in other cases cannot 

 be postulated, unless one thinks of the early prophase as reaching farther 

 back into the interdivision phase of the cell than is commonly supposed. 

 Since it is a fair hypothesis that the chemicophysical changes at the cut 

 surface may produce changes in the blood or the coelomic fluid that might 

 affect the reserve cells, one may question whether the effect of the irradia- 

 tion is upon a humoral activation of this sort or directly upon the reserve 

 cells. It should be possible to determine these points by a combination 

 of irradiation and other techniques. Stolte (57) has recently considered 

 the problem of activation in Dero limosa. Coldwater (9) has made 

 a beginning in this direction by showing that treatment with solutions 

 containing the — SH group increases the rate of regeneration in Tubifex 

 tubifex and that this group presumably functions in the normal regenera- 

 tive process but does not influence the inhibition or retardation of regener- 

 ation by exposure to X-rays. Zhinkin (74) has grafted posterior somites 

 of an irradiated Rhynchelmis upon the cut posterior end of a normal 

 individual, with the result that neoblasts activated by cutting of the 

 nonirradiated worm migrated back into the irradiated transplant and 

 normal regeneration followed. Apparently the irradiated epithelial cells 

 were stimulated, by the presence of the neoblasts, to produce new somites. 



TUNICATA 



In tunicata there are extensive powers of regeneration in solitary as 

 well as colonial species. The histology of this regeneration and of the 

 correlated processes of asexual reproduction, which are known to be 

 essentially similar, has been studied intermittently for many years since 

 the observations of Hjort (26) and Lefevre (36) aroused interest by 

 contradicting the doctrines then current regarding the potencies of the 

 germ layers. Thus in the budding of Perophora, Lefevre found amoeboid 

 blood cells to be the most important factor in the formation of new parts 

 and in the main his observations have been substantiated by the work of 

 later investigators upon budding and regeneration in tunicates. His 

 view was that the fate of any one of these totipotent cells was an out- 

 come of the place of attachment and the stimuli received in that particular 

 region as the bud development proceeded. The origin of these blood 

 cells it was, of course, impossible to establish as direct descendants of 

 the embryonic mesoderm, but there was no evidence that any of them 

 had arisen by dedifferentiation. 



In a recent study of regeneration and reduction in Clavelina, Spek 

 (54) has described certain vacuolated cells as totipotent in regeneration 

 and as arising in a short region of the gut just behind the stomach. 

 Brien (5) has made a good case against these "Tropfenzellen" of Spek 

 as the source of new material during budding and regeneration. Accord- 

 ing to Brien, such cells are nutritive in function and important during 



