INTERNAL FACTORS OF REGENERATION IN ANIMALS 69 



THE CLOSING IN OF CUT-EDGES 



One of the most familiar changes that takes place when a cut-edge 

 is exposed involves the rapid covering over of the exposed tissues. 

 This takes place from the margin of the wound, and a layer of cells, 

 usually the ectoderm at first, covers the surface. The closing in is 

 brought about in many forms by the contraction of the muscles of 

 the outer wall of the body. This seems to be the case in the earth- 

 worm and in the planarian, as well as in other animals, such for in- 

 stance as the starfish, holothurian, etc. But in addition to this 

 purely muscular contraction another process takes place, that is less 

 conspicuous in forms in which the muscles bring about the first clos- 

 ing, but which is evident in forms in which the muscles are absent 

 or little developed. I am able to cite two striking cases that have 

 come under my own observation. When a piece is cut from the stem 

 of tubularia, the ends close in twenty minutes to half an hour. The 

 body wall, the coenosarc, composed of the two layers of ectoderm and 

 endoderm, withdraws a little from the cut-edge of the outer hard tube, 

 or perisarc, that covers the stem, and then begins to draw across the 

 open end. A perfectly smooth, clean edge is formed that advances 

 from all points to the centre, where the final closing takes place. The 

 closing is not due to an arching over of the coenosarc, but the thin 

 plate is formed standing nearly at right angles to the outer tube. 

 This plate is composed of two layers of cells, of which there are a 

 number of rows arranged concentrically between the centre and the 

 outer edge. In the absence of muscle-fibres in the stem, the result 

 cannot be due to a muscular contraction, and even if short fibres 

 existed the transportation of cells entirely across the open end would 

 speak against this interpretation. 1 Since the closing over takes place 

 without any support, we cannot suppose the process to be due to 

 any sort of cytotropic effect. The closing takes place equally well 

 in diluted sea water and in stronger solutions. The method of 

 withdrawal of the cells, as best seen when longitudinal pieces are 

 studied, resembles very much the withdrawal or contraction of proto- 

 plasmic processes in the protozoa, and so far as one can judge from 

 resemblances of this sort, the two processes appear to be the same. 



This closing in of the cut-surface, while a preliminary step in the 

 process of regeneration, cannot, I think, be regarded as a part of the 

 regeneration in a strict sense. That the two processes are not 

 dependent on the same internal factors is shown by the following 

 experiments : If a bunch of tubularia is kept in an aquarium, it will 



1 I have found that the closing in takes place equally well when one per cent of KG is 

 added to the sea water. This salt has, as Loeb has shown, an inhibiting effect on muscular 

 contractility, not, however, on amoeboid movements. 



