REGENERATION 



427 



logical changes during the regeneration of planarians which derives the 

 formative cells by dedifferentiation as well as by division of formative 

 cells already in existence (Fig. 3). According to this concept, the indi- 

 vidual has at the outset a limited number of these cells in its parenchyma. 

 When it is cut for regeneration this initial supply becomes greatly reduced 

 in numbers by migration to the cut surface to form the cap of regenerative 

 tissue. As the regenerative and regulatory changes proceed, more 

 formative cells are produced by dedifferentiation of other cells that are 

 not too highly specialized; and this new crop multiplies and migrates as 



Mit 



DegZ' 





'^;^ 



•«- 



^ 



^,i 





NK 



f'fPi^T^kiij ":': 



Fig. 10. — Effect of radium upon cells of Polycelis nigra. Left — a sagittal section 

 through the cap of regenerative tissue on a tail piece exposed 30 min. to radium, cut imme- 

 diately thereafter, and killed 5 days later. In the cap region there is a very small collection 

 of regenerative cells, some of them showing normal mitoses. Degenerating cells are 

 abundant elsewhere in the parenchyma. Right — a detail from such a section. Da, gut 

 lobe; Deg Z, degenerating cell; Ep, epithelium; Mit, mitoses; NK, normal nucleus; Pa, 

 parenchyma, RK, cap of regenerative cells. {From Weigand, 67.) 



necessary (55, 56). With this picture in mind, Weigand concluded from 

 his observations that the formative cells already in existence were injured 

 by the rays, but that the dedifferentiations by which more formative cells 

 were produced were not interrupted. Only when the dedifferentiating 

 cell reached the state of a formative cell and mitosis began (Lang, 35) was 

 there evidence of effects of the irradiation. The larger numbers of 

 degenerating cells in later stages of irradiates that had been cut, as com- 

 pared with irradiated whole specimens, were interpreted as resulting 

 from formative cells that had arisen by dedifferentiation and then 

 degenerated. The fact that injury was greatest when the cutting was 

 done 2 or 3 days after exposure and the regeneration thus began at that 

 period, he explained by supposing that more of the cells capable of 

 dedifferentiation were mobilized at that time and in an especially critical 

 stage. The fact that a small regeneration cap could be formed after 

 short exposures and the formative cells differentiate normally in the same 

 was explained by supposing that some of them had escaped injury and 

 retained their power of migration. Having once arrived at the cap they 

 seemed more resistant to the rays, since degenerating cells were never 

 abundant in this region; and as the cap differentiated, this resistance 



