l6o T. H. MORGAN. 



closure, disclose several points of interest. The membrane, that 

 forms and advances ring-like toward the center, is relatively thick 

 and is composed of a large number of cells. The slight with- 

 drawal of the coenosarc from the cut end is insufficient to account 

 for the presence of so many cells, and the only interpretation that 

 remains is that the cells of both ecto- and endoderm must be 

 drawn towards the cut surface for some little distance from 

 the ends, although the closure of quite short pieces shows that 

 the result may also be attained by a shorter length being utilized. 

 This involves a greater decrease in thickness of the neighboring 

 coenosarc wall. 



What factors are involved in the closure ? We may suppose 

 that the stimulus of the sea water, or surface tension, or the loss 

 of contact relations of the material, with its concomitant change 

 of tension relations, is the initiatory stimulus. The last seems to 

 me the main factor for reasons given in previous papers. The ten- 

 sion equilibrium lost, contraction follows. The cut edge contracts 

 as a whole, and as a result of the intimate fusion of the outer 

 layer of the ectoderm with the perisarc the diaphragm-like mem- 

 brane develops ; for if separation from the perisarc is first accom- 

 plished a different kind of closure takes place. As the process 

 continues the cells are drawn out bodily over the transversely 

 closing membrane. Despite the rather intimate union of the cells 

 with each other they not only change shape but shift their rela- 

 tions to each other, and while at first a considerable number 

 of cells reach the edge of the closing membrane, their number 

 becomes fewer as the opening gets smaller. 



Our analysis leads to the conclusion that the closure is a con- 

 tractile process of the living substance. 



When closure is completed the relation of the parts to each 

 other is still an unstable one for the end of a piece. This leads 

 to further changes in conformity with the irritable nature of the 

 materials and a new polyp develops in consequence. If our 

 analysis is correct we are led to the view that the essential factor 

 in the closure of the stem is the irritability of the protoplasm which 

 leads it to undergo movements (through its power of contractility) 

 of a peculiar kind. This same process, that we find reduced to 

 its simplest term in the closure of the stem, is the mainspring 



