MEKOGUNY AND REGENERATION IN REXILLA. 



the anterior piece (4, D] had generated a long peduncle, from the 

 anterior end of which the original axial polyp and right lateral 

 one diverged as shown in the figure. 



This case shows with great clearness that very soon after the 

 operation the fragment is plastically remoulded, in a manner 

 somewhat similar to that described by Hargitt and Morgan, 1 in 

 the medusa Goirioncuuis, so that without the formation of new tis- 

 sue the pieces assume a new condition of equilibrium which in the 

 posterior piece brings a lateral polyp into the former position of the 

 axial one. This process takes place so quickly that it seems inad- 

 missible to suppose that it is due to an active process of growth. 

 It gives rather the impression of a mechanical process due to the 

 operation of purely physical factors (tension of the tissue or the 

 like) by which a new condition of equilibrium is restored as 

 nearly as possible like that of an entire colony, and hence repre- 

 senting a case of pure " mechanical regulation " in the sense in 

 which Child has construed the primary process of morphallaxis 

 in planarians. 2 Broadly speaking this process is probably of the 

 same nature as that by which the edges of a cut surface close, 

 though in both pieces the change of form was already complete 

 while the wound was still widely open, as may be seen from Fig. 

 4, B. As such, it cannot be considered as part of the regenera- 

 tive process in the strict sense, 8 indeed this is proved by the fact 

 that the ensuing regeneration gradually counteracts the effect of 

 the initial remoulding. I can, however, find no ground in Mor- 

 gan's own discussions for excluding such a mechanical remould- 

 ing of the old parts from the conception of morphallaxis in gen- 

 eral, and the same ground is taken in the case of Stenostoma by 

 Child, who reaches the conclusion that morphallaxis in this form 

 is "essentially a change in form resulting from differences in 

 mechanical tension in the piece as compared with the whole " 

 ('02, p. 414). The facts of initial morphallaxis observed in 

 Rcnilla fall in very well with Morgan's tension-hypothesis, and 



1 C. W. Hargitt, " Recent Experiments on Regeneration," Zool. Bull., I., 1897. 

 T. H. Morgan, " Regeneration in the Hydromedusa Gonionemns" Am. Nat., 

 XXXIII. , 1899. 



2 C. M. Child, "Fission and Regulation in Stenostoma," Arch. Entwm., XV., 

 2, 3, 1902. 



3 Cf. Morgan, "Regeneration," p. 69. 



