FORM REGULATION IN CERIANTHUS

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Abstract
1. In cylindrical pieces of Cerianthus obtained by two transverse cuts collapse occurs at once and the cut ends begin to roll inward soon after section, finally coming into contact and closing the opening more or less completely. Since little or no transverse contraction of the infolded margins occurs they are thrown into numerous radiating folds and wrinkles. 2. Within two to three days after section a thin membrane formed by the growth of new tissue from the cut surfaces closes the two ends completely. The piece now becomes gradually distended with water, probably owing to the accumulation of metabolic products in the enteron and consequent diffusion of water into this closed cavity. As distension proceeds the infolded margins of the body-wall at the two ends are forced apart by internal pressure and the area occupied by the thin membrane increases. 3. The first step in the regeneration of tentacles is the formation of a slight ridge, the marginal tentacular ridge, on the oral end. This ridge is formed wholly within the tissue of the old body-wall, its formation being accompanied by reduction and disappearance of the muscular layer, disappearance of the pigment and great reduction in thickness. The marginal tentacles first appear as slight upgrowths from the highest—most oral—point of the ridge, one tentacle corresponding to each intermesenterial chamber. The position of the mesenteries is indicated externally on the tentacular ridge by slight furrows which separate the regenerating tentacles from each other. 4. The regenerating marginal tentacles appear at first in a single circle and all usually regenerate with nearly equal rapidity, except in some cases the youngest pair in the growing region. The directive tentacle is usually slightly thicker than the others since the directive mesenteries are somewhat farther apart than the other mesenteries. Rapid increase in length occurs in the marginal tentacles, and the arrangement in about three circles or rows is gradually attained in consequence of the fact that there is not sufficient space on the margin of the disc for all of the tentacles in a single row; some are forced peripherally by the mutual pressure exerted. 5. As the tentacles grow the disc expands and the distinction between the thin membrane of new tissue which first closed the end and the old body-wall with which it was connected disappears completely in consequence of the complete disappearance of the muscular layer, the reduction in thickness, and the loss of pigment in the body-wall of the oral end. 6. The mouth appears after the marginal tentacles, are well established near the base of the directive tentacle, gradually extending along the directive plane across the center of the disc until it is symmetrical. The part of the mouth first regenerated is the region of the siphonoglyph. 7. The labial tentacles do not appear until the marginal tentacles have attained a length of several millimeters. Each tentacle appears as a distinct bud over an intermesenterial chamber, but some intermesenterial chambers are without labial tentacles. 8. After the aboral end is closed by the new tissue this slowly acquires a conical form, protruding from within the wrinkled margin of the old body-wall. The wrinkles on the latter gradually disappear and the pigmentation slowly fades out for a short distance oral to the cut end, this change being connected with reduction and disappearance of the muscular layer as this region of the body-wall becomes involved in processes of growth and redifferentiation in the same manner as the oral end. The aboral end grows out into an elongated conical form at the end of which the aboral pore appears. As the new muscles differentiate in this region pigment stripes begin to appear.

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