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Stages in Regeneration.
The stages in regeneration were studied by the examination of specimens at known intervals after autotomy. Until it was evident that the rate of regeneration in S. mollis is much slower than that found in previously studied species, specimens were killed for examination at 2–4 day intervals. Later the time between examinations was extended to 8–12 days.
During the first 12 days there is little evidence of regeneration visible to the unaided eye. The mesenteric remnants are still approximately the same size as those found immediately after autotomy, and the ruptured anterior end of the cloaca remains unclosed. Histological study shows closure of the ruptured end of the oesophagus. No changes in the mesenteric edge could be detected up to this stage.
Specimens examined after 12 days' regeneration showed a progressive increase in the distance from body wall to mesenteric edge in certain regions. Greatest extension occurs around the angle formed by the crossing of the dorsal mesentery over the right dorsal interambulacrum before it travels forward as the lateral mesentery. Extension in this angle lessens the distance of the free mesenteric edge by decreasing the angles of the loops, resulting in a more direct connection between oesophagus and cloaca. If the distance from the anterior end of the lateral mesentery to the posterior point of origin of the dorsal mesentery (x - x”., Fig. 1) is taken as unity, it is found immediately after autotomy that the mesenteric remnant from its point of origin at the angle (x′ - x″., Fig. 1) traverses only 10% of this distance (Figs. 1 and 2). Thus the free edge of the mesenteric remnant travels posteriorly and then forward for 90% of this distance.
The extent of the growth of the mesentery after various intervals of regeneration is shown in Fig. 2 for 18 specimens which had undergone autotomy at the same time, and regenerated under identical conditions. After 17 days' regeneration (Fig. 3) the mesentery extends forward 20–25% of the distance, reaching 65% after 33 days' regeneration (Fig. 4), 75% in 41 days' regeneration (Fig. 5), and by 65 days' regeneration it has elongated to form a pocket-like structure, of which the lip bearing the regenerating alimentary canal is parallel to the anterior end of the ventral loop. When this stage is reached there is no longer any looping of the free edge in its traverse from the dorsal to the ventral mesentery (Figs. 3–6). The connection from one to the other is then in a straight line. From Fig. 2 it can be seen that the rate of increase in depth is almost con-
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stant(except for one specimen) instead of showing a logarithmic curve as found in most growth processes. This is probably due to extension occurring in one plane as a sheet which does not increase in thickness. Growth is, therefore, restricted to two dimensions. At the mesenteric edge, the slight thickening which is the primordium of the regenerating alimentary canal, is not sufficiently large appreciably to slow down the extension of the mesenterie sheet.
Fig. 2.—Growth increment of X′–X″ (Fig. 1) as a percentage of X–X”., during regeneration.
At the same time there is a lesser increase in depth of the mesentery in the angle between the lateral and ventral loops. This causes a posterior movement of the position at which the free mesentery edge crosses from the lateral to the ventral loop. There is no increase in depth of the mesentery along the right ventral interradius from the level at which the free edge crosses from the lateral mesentery, to the junction with the cloaca.
Changes in the depth of parts of the mesentery, continuously decrease the length of the free edge from oesophagus to cloaca. This gradually eliminates the angles of looping, which are so characteristic of the alimentary canal and its mesenteric attachments in normal animals. After 65 days' regeneration there is almost a straight course from oesophagus to cloaca.
There is a series of changes along the free mesenteric edges during the alterations in lengths of the mesenteries. The normal mesentery consists of a thin connective-tissue sheet covered by an epithelial layer continuous with that lining the body cavity. The lining epithelium on each side of the mesentery meet across the torn edge after 15 days' regeneration, and fuse in the mid-line. Mesenchyme cells between the epithelial layers increase in number to form a slight thickening along the mesenteric edge. It is from this collection of mesenchyme cells that the alimentary canal is regenerated.
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Figs. 3–6.—Four stages in regeneration showing the regions of growth of the mesentery, and the restoration of organs.
A.C., alimentary canal; CL., cloaca; D.M., dorsal mesentery; L., alimentary canal at the stage where a lumen is forming; L.M., lateral mesentary; O., incompletely closed rupture of cloaca; OES., remnant of original oesophagus; R.T., developing respiratory trees; S., alimentary canal appearing as a solid cord of cells; T.C.V., transverse connecting vessel; V.M., ventral mesentery; V.I.V., ventral intestinal vessel.