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Discussion.
The relationships of the organs found after regeneration in the three specimens examined in the present study may be compared with those found by Bertolini (1932), in Holothuria tubulosa after 60 day's regeneration. In the latter, a thin transparent tube grows posteriorly from the oesophagus and crosses the coelomic cavity to the mesentery which previously held the intestine, omitting all the mesentery which formerly held the loop of the stomach. At the same time another thin tube with a blind end starts from the cloaca and extends forward along the mesentery which formerly held the last portion of the original intestine. After about 60 days the two tubes meet and unite to form a single tube extending from oesophagus to cloaca. Holothuria floridana and H. impatiens also show the growth of rudiments from the anterior and posterior ends to meet and form the regenerating alimentary canal, but in these species the rudiments involve the edge of the mesentery throughout their entire length (Kille, 1936). Scott (1914) noted that the regenerating alimentary canal of Thyone briareus was situated along the mesenteric margin, but Kille (1935) was the first to show that it was developed by proliferation of the mesenteric tissue and that only the intestinal epithelium was derived from two centres at opposite ends of the animal. To test the possibility of the regenerating alimentary canal growing across the coelom from one section of mesentery to another in Thyone, he removed sections of mesentery, but found that they rapidly redeveloped from minute fragments which remained close to the body wall, and a typical regeneration followed.
The condition found in the three specimens of Stichopus mollis thus most closely resembles that found in Holothuria tubulosa after 60 days' regeneration. In both species a certain region of the original mesentery is avoided and the alimentary canal follows an almost direct course from the oesophagus to the cloaca.
The stages in reaching this condition, however, differ in the two species. While it is reached in Holothuria tubulosa by the union of rudiments growing towards each other from anterior and posterior ends. this is not the case in the present specimens of Stichopus mollis.
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The alimentary canal of a specimen examined after 44 days' regeneration tapered markedly posteriorly, with the narrowest diameter and least histological differentiation just anterior to the cloaca. If there had been a forward growth from the cloaca the diameter and state of differentiation in this region would be as least as great as farther anteriorly along the mesenteric edge, and would be expected to be actually greater. The condition found in the specimens studied showed clearly that there had been no forward growth of a tubular rudiment from the cloaca. All three specimens showed an abrupt decrease in diameter from the oesophageal remnant to the regenerating alimentary canal, with no gradual transition between the two regions. None of the specimens gave evidence of posterior growth of the regenerating alimentary canal from the remnant of the oesophagus. In normal regeneration in Stichopus mollis, the whole of the regenerating alimentary canal is formed along the mesenteric edge from mesenteric tissue, without any contributions from remnants of the alimentary canal at the anterior and posterior ends. There is no evidence to suggest otherwise in the three specimens examined in this present study.
While the connection between the anterior and posterior portions of the alimentary canal in Holothuria tubulosa is made after the growth of a rudiment across the coelomic cavity, the alimentary canal in S. mollis follows the mesenteric edge along its length over a bridge formed by the adhesion of part of the dorsal mesentery to the ventral mesentery.
The section of the mesentery omitted from the course of the regenerating alimentary canal in the three specimens of S. mollis is the region which in normal regeneration shows the greatest increase in depth as it grows forward to eliminate the angle between the edge of the dorsal and lateral mesenteries (See Fig. 1.). After 60 days' regeneration, the mesentery in this angle normally undergoes a tenfold increase in depth from its point of attachment at the body wall, to the free edge supporting the regenerating alimentary canal. In these three cases, where this region did not have regenerating tissue along its edge, there was no evidence of any increase in length of the mesentery after 50 days' regeneration. This may indicate that some interaction between the mesentery and regenerating tissue at its edge causes an extension of the mesenteric sheet in this region. It seems highly probable that the proliferating tissue of the regenerating alimentary canal, normally present in this region, develops a local growth stimulator. In its absence. no stimulation for increase in depth of the mesentery occurs.
The excluded segment of the mesentery shows no regenerating tissue along its edge. This could be due to loss of potency of the mesenteric edge, or a lack of some inducing agent necessary for proliferation and differentiation in the region omitted. As regeneration can occur in all specimens of Stichopus mollis, including the largest sizes obtainable, the mesentery appears to retain its potency throughout life, so it is to be concluded that regeneration was absent because no inducing agent was present.
The inducing agent may be absent because an anterior-posterior polarity has already been established in a direct course from the
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oesophagus to the cloaca. In normal regeneration the new tissue follows an S-shaped course initially with some of the anterior parts of the alimentary canal situated behind other parts which are morphologically posterior. The state of differentiation of the regenerating alimentary canal then bears no constant relationship to position along the mesenteries edge (Dawbin, 1949). In later development the extension of the mesenteries results in a straightening of the alimentary canal towards the anterior-posterior axis. At this stage the regenerating alimentary canal tapers from a more advanced condition of regeneration anteriorly, to a less advanced condition posteriorly, and continuous anterior-posterior gradient is then first apparent, and has been reached slowly by activity involving the total length of the mesenteric edge.
A direct route for the regenerating alimentary canal is provided along the anterior-posterior axis at a much earlier stage in the cases of adhesion of the dorsal to the ventral mesentery, and it provides the substratum for a gradient of activity in the axis of polarity of the whole animal. Once this gradient is established, it is suggested that the mesenteric edge not included in the axis is then outside the gradient field of organising activity, so proliferation and differentiation of tissue is not induced in this region.