FORM REGULATION IN CERIANTHUS. 255 



The directive tentacle, which corresponds to the chamber between 

 the two directive mesenteries, is slightly thicker than the other 

 tentacles in consequence of the fact that the directive mesenteries 

 are somewhat farther apart than other mesenteries. As regards 

 the arrangement of the marginal tentacles it will be seen that they 

 are no longer in a single row, but some appear as if they were being 

 crowded out of the row, owing to lack of space. In all proba- 

 bility that is what is occurring. As the tentacles increase in size 

 there is not sufficient space for them in a single row upon the 

 margin and some are pushed out, probably in most cases 

 peripherally. 



Fig. 21 is a schematic figure of one half the body after longi- 

 tudinal section in the directive plane, the directive tentacle being 

 on the left of the figure. The stage of regeneration is about the 

 same as that of Fig. 20. As compared with the earlier stages 

 (e. g., Fig. 19) several points of difference are to be noted : the 

 marginal tentacles are longer, the difference in thickness between 

 the reduced body wall of the tentacular region and the thin new 

 tissue across the disc has completely disappeared ; the reduction 

 and disappearance of the muscular layer extends further aborally 

 than before ; the regeneration of the mesenteries has advanced ; 

 and finally there is a minute mouth, which, as was evident from 

 Fig. 20, is not centrally placed, but lies near the base of the 

 directive tentacle. 



One or two points of importance as regards the regeneration 

 of the mesenteries may be noted. In the normal animal a slight 

 furrow, which appears as a faint longitudinal line on the surface 

 of the entoderm, extends aborally from the aboral end of each 

 mesentery. In a piece cut from the middle region many of the 

 mesenteries lie wholly oral to the cut and so are not present in 

 the piece, but most of the furrows, extending aborally, are visible ' 

 in the piece. The mesenteries regenerate along these furrows. 

 Whether regeneration of a particular mesentery aboral to the 

 end of the furrow representing that mesentery is possible has 

 not yet been determined. The point to which it is desired to 

 call attention here is that the mesenterial regions are determined 

 for some distance posterior to each of the mesenteries them- 

 selves. 



