CLOSURE OF TUBULARIAN STEMS. 



155 



METHOD OF CLOSURE. 



As the result of the splitting, the stems were divided into part- 

 cylinders of various sizes, some almost complete, others with a 

 mere strand of coenosarc ; while in some, an oblique split pro- 

 duced pieces nearly complete at one end and very small at the 

 other. The method of closure seems to be independent of the 

 size of the piece, the same general process being followed in all 

 pieces. The cut leaves a free edge of. both ectoderm and endo- 

 derm. In sections of pieces which had been killed five, ten or 

 fifteen minutes after the splitting, the endoderm cells had rounded 



up over the ectoderm, probably as a result of disturbing the 

 osmotic pressures (Fig. A). The first step towards closure of the 

 tube is the pushing of the ectoderm over the endoderm (Fig. B). 

 Whether this is accompanied by any activity on the part of the 

 endoderm cannot be stated definitely, but it seems probable 

 from some sections and from what occurs in later stages, that 

 the endoderm cells change their shape, their outer ends next the 

 ectoderm becoming broader and the inner ends becoming pointed, 



