FORM REGULATION IN CERIANTHUS. /I 



growing over the opening is in two parts which are advancing to 

 meet near the middle. The concave free margin of both portions 

 is noticeable. The dotted lines represent various stages in the 

 growth of the new tissue. It is evident that it appeared first in the 

 angles where two cut surfaces were in contact or very closely ap- 

 proximated. In C. solitarius closure by new tissue of only very 

 much smaller pieces is possible. I believe the explanation of this 

 difference is to be found in the different quality of the thin mem- 

 brane of new tissue in the two cases. In C. meuibranaceus it is 

 much thicker, more resistant, and less easily ruptured than in C. soli- 

 tarius. The new tissue arises at a region where the cut surfaces are 

 close together and may extend from this to regions wHere they are 

 more widely separated. As was shown above, it exerts a certain 

 degree of tension on the parts connected by it. As the distance 

 between the cut surfaces increases a point may finally be reached 

 where the tension is equal to the cohesive power of the tissue ele- 

 ments. Beyond this the new tissue cannot extend. In C. solitarius 

 this limit is attained with a slight separation of the cut surfaces, 

 while in C. membranaceus the new tissue is capable of resisting 

 much greater tension and so of extending over wider spaces. 



The membrane extending between the two cut surfaces may be 

 compared with a fluid film bounded by lines diverging at an acute 

 angle. The film extends a certain distance from the apex of the 

 angle, this distance being determined with a given fluid by the 

 size of the angle. The free margin of the film is always concave 

 toward the opening of the angle. So long as the relation between 

 cohesion and adhesion remains the same and the angle does not 

 change the film can never extend beyond a certain point, since 

 the surface-tension will cause rupture. As the angle and surface- 

 tension decrease or as the adhesion increases the film will spread. 

 If the*arms of the angle are sufficiently pliable or capable of 

 movement they may be drawn together by the surface-tension of 

 the fluid, and thus permit further extension of the film. 



In Cerianthus the thin membrane of new tissue which may be 

 compared to the fluid film, arises at the apex of the angle, /. e., 

 where the two cut surfaces are in contact. The membrane ex- 

 tends along the diverging surfaces to a certain point. Its free 

 margin is always concave (Fig. 31, also Figs. 68 ; '03^). The 



