REGENERATION 429 



variations in head forms regenerated are the same type as those produced 

 by other physical and chemical agents. It is no more necessary to assume 

 the selective action of X-rays on formative cells than it is necessary to 

 assume selective action of other physical and chemical agents which alter 

 head frequency. The first apparent effects of X-rays, hke various other 

 agents, seems to be not on special formative cells but upon nonspecific 

 protoplasmic factors upon which head development depends." 



With regard to these statements it can be remarked that while the 

 work of Curtis and his students, like that of many other investigators, 

 has shown the importance of such cells in the regeneration of planarians, 

 the "formative cell theory" is the theory of Weismann and not of any 

 later investigator. It seems abundantly established that the so-called 

 formative or regenerative cells function in planarians in the formation 

 of new parts during regeneration and that such cells give rise to the germ 

 cells. As shown by Fig. 3, the immediate origin of these cells is not so 

 obvious. What can be claimed with respect to irradiation is that radium 

 and X-rays destroy these cells with embryonic potencies, in the same 

 manner that these rays destroy embryonic cells in many organisms 

 according to the law of Bergonie and Tribondeau. The effect appears in 

 the cells soon after exposure, whether due to "direct hits" or to indirect 

 effects upon the organism as a whole that quickly affect these cells 

 without affecting others in any recognizable manner. In the absence of 

 evidence for either possibility, the later effects of deformation as described 

 by Weigand and the cytolysis described by Meserve and Kenney may 

 be explained as well by the toxic action of material arising from degenera- 

 tion of formative cells, and accumulating in the parenchyma, as from more 

 direct effects of the irradiation. 



It seems from these studies that the technique of irradiation provides 

 a check upon the investigations that have shown the importance of 

 formative cells in the regeneration of planarians. The weakness of such 

 results with radium and X-rays lies in the disputed origin of the formative 

 cells, in the difficulty of recognizing formative cells as such, and in the 

 fact that individual planarians, after becoming wholly incapable of 

 regeneration as a result of such irradiation, cannot be maintained ahve 

 and in an otherwise normal state for a long period. As with hydras, 

 one should obtain individuals that possessed no formative cells and could 

 not regenerate, but lived indefinitely without other abnormalities. If 

 the formative cells are a persistent embryonic stock, necessary for other 

 cell replacements and for sexual reproduction as well as for regeneration, 

 it would perhaps be too much to expect that the individual thus deprived 

 of these cells could live out a normal span of life, whatever that may be 

 for a given planarian. It might be expected, however, to five much 

 longer than the periods indicated, and one might hope to determine the 

 lethal conditions if it died prematurely. 



