466 



WAYLAND M. CHESTER 



The region of union at the proximal end is similar in extent and 

 character. Around this region of union is one in which the cells 

 of the ectoderm are obUque to the surface of the layer. Nettle 

 cells (fig. 8, nm'cys.) and gland cells {cl. muc.) occur here, but the 

 nerve layer and mesoglea are not found. This region is very 

 narrow at the proximal end. 



ms'gl. 

 st.n. 



-nnvcys. 



cj.muc. 



8 



Fig. 7 Camera outline of longitudinal section of excised tentacle whose tip 

 was removed at time of excision. Four days after excision. X 30. 



Fig. 8 Distal end of tentacle exhibited in figure 7, showing the region of clos- 

 ure of the tentacle. X 190. ms'gl., mesoglea; st. n., nerve layer; co'jct., region 

 of ectodermic union; nm'cys., nematocyst; cl. rmic, mucus cell. 



Some grafting experiments were made to see if the polarity is 

 reversible. The Metridium was kept in chloretone until it no 

 longer responded to touch. A tentacle was then cut off near the 

 base and its tip removed. Through the lumen of the tentacle 

 fragment was thrust a bristle, by which it was carried to an 

 attached tentacle which had been cut at such a level that its cut 

 edge was of nearly the same circumference as the edge to be grafted 

 upon it. The apposition of the edges was difficult, because the 



