Studies on Regulaticn. 541 



tension resulting from progressive movement (Child, '04a) which 

 antagonizes the conditions at the cut surface. Thus, as the animal 

 advances, the cut surface is frequently stretched to a considerable 

 extent. The position represented in Fig. 57 is betw^een the two 

 extremes and represents the usual form during undisturbed pro- 

 gression. In the rapid progressive movements following strong 

 stimulation the body often becomes almost straight and on the 

 other hand when at rest or moving very slowly is more curved 

 than in the figure. 



In the left piece, however, rapid progression does not occur and 

 the body adheres only very slightly; consequently the body is not 

 subjected to longitudinal tension and the cut surface is not 

 stretched but continues to contract indefinitely, like the anterior 

 cut surface in pieces without ganglia. These cases only confirm 

 the opinion already expressed that ordinary muscular contraction 

 has little or nothing to do with the contraction of cut surfaces; it 

 may be that muscles which have been cut and are thus free at one 

 end gradually shorten, but this process is by no means the same 

 as muscular contraction in the ordinary sense. Even after long 

 periods the animals are unable voluntarily to straighten their 

 bodies; nothing but mechanical tension in the longitudinal direc- 

 tion will accomplish this end — and this fact is one of the most 

 striking proofs of the influence of mechanical tension upon form 

 in these animals. As has been suggested elsewhere the contrac- 

 tion is probably due to surface tension, capillarity, elasticity and 

 other mechanical factors, which are visibly effective in this way 

 only in the absence of the mechanical conditions which under 

 normal circumstances antagonize them. Doubtless also the re- 

 duction in functional activity of the parts is accompanied by more 

 or less reduction in size. 



Comparing now the regeneration of the two pieces it is evident 

 that it is much greater in amount in the right piece (Fig. 57) than 

 in the left (Fig. 58). The thickness of the new tissue is about the 

 same in both, but in the right piece its width and length are both 

 greater than in the left piece. Intestinal branches are growing, 

 out into the new tissue anterior to the pharynx in the right piece, 

 but none are visible in the left piece. The new tissue, especially 



