REGENERATION 435 



data by measuring the regenerated outgrowths or countmg their somites. 

 These problems have been further attacked in related material by trans- 

 plantation experiments (65, 42); and by experiments showing that the 

 — SH group stimulates regeneration and is apparently essential for 

 growth during normal regeneration (9). Results have also been obtained 

 by irradiation and this technique gives further promise. 



In Tubifex tuhifex following an exposure to X-rays of 7125 r, Stone 

 (58) found that posterior regeneration never occurred (Figs. 11 and 12). 

 There was only the formation of a small knob by rearrangement of 

 existing tissues without cell proliferation or differentiation. After this 

 healing additional somites could be removed at intervals with repetition 

 of the healing and knob formation. The inhibition of regeneration 

 continued as long as the worms survived in the cultures, in one experi- 

 ment 147 days, which was as long as the normal uncut worms were main- 

 tained. The irradiated specimens thus behaved as though "castrated" 

 against regeneration. In contrast to this, control worms from which 

 as many as 32 posterior somites were removed regenerated as many new 

 ones in about 32 days. 



In specimens cut for posterior regeneration Stone found no neoblasts 

 in any of the Tuhifex that received the 7125-r exposure and consequently 

 none of the histological changes dependent upon neoblast growth, 

 migration, and differentiation. In like manner the changes in the epi- 

 thelia which occur in the normal regenerate were inhibited. A similar 

 inhibition of anterior regeneration and of its cell multiplication in the 

 epithelia was obtained by Stone with exposures to X-rays of 9000 r 

 (Fig. 13). Turner (63, 64) has obtained comparable results with Lum- 

 briculus inconstans, and Zhinkin (72, 73) with L. variegatus; Rahn (49) 

 has found that the neoblasts of naids are not present after irradiation 

 with radium; and Stone (60) obtained inhibition of regeneration in 

 Euratella chamherlin by exposures with radium. There seems to be no 

 question as to the establishment of such an inhibition in the annulata 

 and the correlated structural and functional changes in cells as described. 



By means of the irradiation it is possible to eliminate the neoblasts, 

 as though one cut out parts of the organism by a surgical operation. 

 Then it is possible to follow what happens in regeneration; indeed, the 

 embryologist has long proceeded in the same manner with analogous 

 experimental operations that have greatly extended our knowledge of 

 development. The results in this phylum are perhaps more satisfactory 

 than in any of the other cases cited previously, because of the clear 

 histological picture that is presented and the precision of the changes 

 that can be induced by the irradiation. The problem remains of how 

 both the reserve cells of the mesoderm and the epithelial cells are affected 

 to render them incapable of their normal behavior following removal of 

 a part of the individual, that is, why these cells are no longer activated. 



