FORM REGULATION IN CERIANTHUS. 2/7 



distension of the piece. However the fluid may enter, the dis- 

 tension occurs in every case, and in three or four days after 

 section (in summer) the pieces are subjected to a considerable 

 degree of internal pressure. 



The pressure of the water being exerted in all directions tends 

 to stretch the body-wall. The thin delicate new tissue at the ends 

 is the weakest part of the body-wall and therefore most effected 

 by the stretching. After closure of the ends by new tissue and 

 before distension has occurred, the new tissue is either invisible or 

 scarcely visible from the exterior. The inrolled edges are closely 

 approximated and all that can be seen of the new tissue in any 

 case is the part filling in the spaces between folds and wrinkles 

 of the cut surface. Moreover, if the inrolled margins of the 

 body-wall be spread apart even with the greatest care at this 

 stage the new tissue does not stretch to any great extent but is 

 ruptured even by slight tension. Under these conditions the cut 



FIG. 3. FIG. 4. 



end presents somewhat the appearance of Fig. 3. The piece 

 is collapsed, the margins are much folded and the only new 

 tissue visible is the small area between the inrolled margins 

 represented in the figure by the stippled portion. 



These facts serve to introduce the chief point in the present 

 consideration, viz., this : so long as there is no tension exerted 

 upon this new tissue closing the ends it does not increase in 

 surface-area, but mere differentiates into the typical structure of 

 the body- wall. In other words the ends of the piece have no 

 power in themselves to spread apart and thus initiate the forma- 

 tion of a new disc. What occurs in the typical course of regen- 

 eration is shown in Fig. 4. The inrolled margins are grad- 

 ually spread apart while the thin membrane of new tissue uniting 

 them the stippled area in the figure --increases in area as 

 they separate. 



