Studies on Regulation. 483 



b. Discussion of the Experiments. 



These three pairs of experiments at three different levels of the 

 body all afford similar results. In all the series containing the 

 ganglia regeneration is quantitatively more complete than in 

 those where the ganglia are absent. Moreover the difference is 

 greatest between Series 73 and 82, is less but still considerable 

 between Series 78 and 79, and is only slight between Series 80 and 

 81, /. e., the difference decreases with increasing distance of the 

 cut surface from the anterior end. And finally, there is in general 

 a decrease in the absolute amount of regeneration in all series with 

 approach of the cut surface to the posterior end. 



In order to forestall the possible objection that differences in 

 thickness in the dorso-ventral dimensions of the new tissue have 

 not been taken into account it should be said that frequent ob- 

 servations upon this point showed no marked difference in thick- 

 ness, though usually the new tissue in pieces without ganglia was 

 not as thick as in the others, probably because the cut surface 

 undergoes greater contraction dorso-ventrally as well as in other 

 directions in such pieces without ganglia. 



In all three cases the pieces without ganglia are smaller than 

 the others, since the whole head was removed with the ganglia, 

 but only in the first case (Series 73 and 82) is the difference in size 

 very great; here the pieces without ganglia are only about half the 

 size of the others. As a matter of fact, however, the differences in 

 size do not appreciably affect the result, as numerous experi- 

 ments have shown me. Other things being equal a smaller piece 

 becomes exhausted and dies sooner than a larger piece, but 

 except in the case of minute pieces both live several months. 

 Moreover, in the above experiments, the smallest pieces without 

 ganglia (Series 73, Figs. 9 and 10) show more regeneration than 

 either of the other series without ganglia (Series 79, Figs. 17 and 

 18; Series 81, Figs. 26 and 27) and the same is true of the pieces 

 with ganglia. The long pieces with ganglia of Series 80 (Figs. 24 

 and 25) show less regeneration than do the small pieces without 

 ganglia of Series 73 (Figs. 9 and 10). It is evident that the 

 differences in size of the pieces cannot account for the results of 

 these experiments. 



