458 WAYLAND M. CHESTER 



The endoderm lags behind the ectoderm in this closing process, 

 more so in some tentacles than in others. Its region of advance 

 is marked by a thinner layer of cells. The mesoglea is bent in by 

 the inrolling process (fig. 7), but in the immediate region of the 

 closure no mesoglea is found in any sections. In the region where 

 ectoderm and endoderm are advancing beyond the mesoglea, 

 the muscle and ganglion cells do not appear, but the nettle cells 

 and gland cells are abundant, except within a very limited region 

 at the immediate edge of the advancing layers, where the cells 

 are shorter and irregular in shape and the cell walls are not 

 prominent. Here nettle cells and gland cells do not occur. 



The unequal tension of the layers seems to be a large factor in 

 the approximation of the cut edges ; but other causes also work to 

 carry the ectoderm and endoderm centripetally across the cut end. 

 To a large extent this is done without great change in the layers. 

 Only very near the advancing edge are conditions seen which more 

 nearly suggest a migration of individual cells ; but in this migration 

 the more specialized cells take little or no part. 



In the normal closure of the distal end of an attached tentacle 

 stump, the cut edges are approximated largely by the action of 

 the sphincter, and closure undoubtedly occurs without great 

 migration of individual cells or changes in the relative position of 

 cell layers. Sections of such a tentacle stump, the animal having 

 been kept in normal sea water twenty-four hours after the tentacle 

 was cut, show, in contrast with the conditions seen in the closure 

 of a detached fragment (fig. 8), that the cell layers in the region of 

 closure are not shallow and irregular, and that both mesoglea and 

 muscle cells are normally distributed. 



When the tentacle was severed obliquely, the closure took place 

 in the same way as when the cut was directly across it. If the 

 oblique cut extended completely through the tentacle, or nearly so, 

 the result was the formation of a nipple like that described, except 

 that its distal surface was oblique to its axis. A sphincter formed 

 even if the cut included no more than half the transverse section 

 of the tentacle. When the cut extended less than half way across, 

 there was a bend of the tentacle at the place of injury and a slight 

 contraction of the muscles near the cut. 



