FORM REGULATION IN CERIANTHUS. 251 



between the intermesenterial chambers, these being now filled 

 with water and under pressure. Here and there, however, folds 

 without such significance occur, and moreover some of the 

 chambers are so situated on the infolded margin that they are 

 more widely open and thus expand more in consequence of the 

 pressure than others, hence the irregularity in form and size of 

 these crenations. 



In Figs. 13 and 14 are shown resoectively the oral and aboral 

 ends of the body-wall at the stage where the infolded portions 

 begin to separate. The thin membrane closing the end is shown 

 as a black line. It consists, of course, of ectoderm and ento- 

 derm, but the muscular layer does not extend into it. 



THE FORMATION OF THE MARGINAL TENTACLES AND Disc. 



Within the first day or two following the closure of the ends 

 and the distension of the piece with water the changes leading 

 to the formation of the characteristic organs of the oral end be- 

 gin. In pieces cut from the middle region of the body the full 

 number of mesenteries is not present, since some end anterior to 

 this region. Regeneration of mesenteries occurs, though the 

 number of mesenteries in a regenerated oral end from the middle 

 region of the body is somewhat less than the number originally 

 present at the oral end of the individual from which the piece is 

 taken. This point will be considered at another time. It is suffi- 

 cient for the present purpose to say that the whole oral end of 

 the piece becomes divided into intermesenterial chambers, in the 

 manner characteristic of the species, by the regeneration of new 

 mesenteries, at first very short, between the longer mesenteries 

 which are present in the piece. Attention was called above to 

 the crenation of the infolded margin in correspondence with the 

 position of the mesenteries (Fig. I i). 



The first marked change following the closure of the end is 

 the appearance of a slight ridge on the infolded margin of the old 

 body-wall as shown in Fig. 15. The ridge is wholly confined to 

 the tissue of the original body-wall, the thin membrane which 

 closes the end playing no part in its formation. The crenations 

 become more distinct and extend in many cases from the margin 

 of the old body-wall over the ridge, as the regeneration of the 



