112 C. M. CHILD. 



many pieces including the two which possess special interest at 

 present, viz., that the aboral end of the oesophagus, although 

 not a cut surface is capable of union with a cut surface. It is of 

 interest in this connection to note that nowhere else in the body 

 does the body-wall terminate in a free margin. This free margin 

 behaves like a cut surface and may like any cut surface give rise to 

 new tissue after union with another cut surface (see Child, '04^). 

 In cases where these pieces succeed in rolling up longitud- 

 inally so that the longitudinal cut surfaces of body-wall and 

 those of the oesophagus unite, a more or less typical mouth and 

 oesophagus result from the union and regeneration may proceed 

 in an almost typical manner. Usually, however, the oesophagus 

 unites with the body-wall as in cesophageal pieces (Child, '04^), 

 either on all sides or partially. Aboral union between oesoph- 

 agus and body-wall is made possible by the bending orally of the 

 aboral part of the body-wall so that aboral end of the oesophagus 

 and aboral cut surface of the body-wall unite. In all cases 

 where union between oesophagus and body -wall occurs elsewhere 

 than at the oral end a closed piece without a mouth is the result 

 and a mouth cannot regenerate. Since there is no communi- 

 cation between enteron and exterior in such pieces it might be 

 expected that they would behave as regards tentacle-regenera- 

 tion like the oesophageal pieces (Child, '04^) i. e., that they 

 would become slightly distended at first and regenerate small 

 tentacle-buds but would afterwards collapse and the tentacles 

 would be reduced. As a matter of fact, however, these pieces 

 though never approaching pieces which possess mouths often 

 show a far greater power of tentacle-regeneration than cesoph- 

 ageal pieces. They frequently regenerate tentacles 56 mm. in 

 length, /. e., twice as long as those of the best cesophageal 

 pieces. They may remain distended for a month or more but 

 sooner or later collapse occurs and the tentacles undergo reduc- 

 tion. Since these pieces are completely closed like the oesoph- 

 ageal pieces they can become distended only as fluid passes 

 through the body-wall ('04^). The important point is that the 

 distended condition continues for a much longer time than in 

 cesophageal pieces. The reason for this difference is probably 

 to be found in the fact that pieces cut as in Fig. 16 contain not 



