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the mesenteric edge, occurring least frequently along the ventral mesentery. After 25 days' regeneration an unlined lumen commences to appear in parts of the previously solid cord of cells. The lumen does not necessarily appear in those parts of the regenerating alimentary canal which have reached greatest thickness. From Table II it can be seen that in Specimen 68 a lumen had appeared at the anterior end of the ventral mesenteric edge in a cord of 0·3 mm. diameter, while the more posterior region of greater diameter was still solid. Specimen 78, examined after 26 days' regeneration, was unusual in having the anterior portion of the regenerating alimentary canal merging with, and reaching almost the same diameter as, the original oesophageal remnant. The most anterior portion showed well differentiated layers such as are found in the normal oesophagus. This was followed by a region showing an unlined lumen amongst the mesenchyme cells and absence of a muscle layer, then by a solid cord of mesenchyme cells which was of the same diameter as that showing normal layers. The diameter of the solid cord in Specimen 78 was much greater than that found in many specimens which were developing a lumen, or even showing differentiation of the cells lining the lumen, e.g., Specimen 87 after 41 days' (Table II) regeneration. Specimen 67 after 33 days' regeneration shows this even more clearly. The thickest portion of the regenerating alimentary canal (1·5 × 0·4 mm.) was still solid, although a lumen was commencing to appear in a region of only 0·2 mm. diameter. The lumen appears without any constant relation with the position along the regenerating alimentary canal, or with its diameter.

Differentiation of the mesenchyme cells lining the lumen was found only in specimens examined after 40 days' regeneration. Specimen 87, examined after 41 days' regeneration, showed a region at the posterior end of the dorsal mesentery which had developed a lumen lined by a well-developed inner epithelium. It commenced at the posterior end of the dorsal mesenteric edge as a cavity in a cord of 0·4 × 0·15 mm. and continued across to approximately half-way up the lateral mesentery, increasing in diameter to this point where the cord had reached 1·4 × 0·3 mm. Along this segment the lumen was lined by a very well-developed inner epithelium (Fig. 7). Farther posteriorly the regenerating alimentary canal constricted sharply to a much smaller diameter, and continued as a solid cord of cells only 0·1 mm. in diameter. The lumen and its lining cells had appeared in a median segment, with only a solid cord of cells connecting it to the oesophagus anteriorly and the cloaca posteriorly.

After 50 days' regeneration the alimentary canal is of greater diameter along its whole length. In some specimens the development of a lumen and the differentiation of its lining epithelium occupied a greater length of the alimentary canal. In others, e.g., Specimen 80 (Table II), the lumen was appearing in all regions of the alimentary canal, but showed strands of mesenchyme cells extending across the lumen and dividing it into several adjacent spaces instead of forming a single cavity. In spite of its increased diameter, the differentiation of a lining epithelium had not become evident.

Fig. 7.—Transverse section of the differentiating alimentary canal.
C., “wandering” cells; C.T., connective tissue; I.E., inner epithelium; L., lumen; M., distal region of mesentery; O.E., outer epithelium; V.I.V., developing ventral intestinal vessel.

An extensive lumen lined by a well differentiated lining epithelium was found in all specimens (except one) examined after 60 days' regeneration. Large irregularities in the diameter of the gut and its state of development were no longer apparent. The increase in diameter of the regenerating alimentary canal in the anterior region was sufficiently great to make the point of junction between it and the original oesophageal remnant difficult to locate. The lumen from one to the other was continuous. Histological study showed that the muscle layer in the regenerating alimentary canal had not yet formed while that in the original oesophageal remnant was distinct. Posteriorly the regenerating alimentary canal contained a continuous lumen until it reached the portion suspended by the anterior third of its ventral mesentery. There was a gradual decrease in diameter to this point, from which it extended posteriorly as a solid cord of cells to a point just before its junction with the cloaca. A slight dilatation may occur a few millimeters before the cloaca with a lumen continuous with the cloacal chamber.

A continuous lumen lined by an inner epithelium from the oesophagus to the cloaca was found for the first time in a specimen which had regenerated, for 80 days. The alimentary canal had still not reached the same diameter along its whole length, but showed a gradual decrease from 2·0 × 1·5 mm. in size anteriorly to 1·2 × 1·0 mm. along the posterior half of the ventral mesentery. There was an enlargement to 2·2 × 2·0 mm. at the region crossing from the anterior end of the lateral mesentery to the ventral mesentery. In normal animals this portion of the mesentery suspends the junction of the small and large intestine. Owing to the extension of the mesenteries during regeneration and the shortening and straightening of the alimentary canal regenerating along its edge, the gut now follows an almost straight course from

oesophagus to cloaca, and this region now represents its anterior third to half. It seems likely, therefore, that this enlargement represents the commencement of the stomach, which has altered its relative position following the mesentery rearrangement. The extensive folding of the lining epithelium characteristic of the normal stomach had not developed sufficiently in any specimen up to 145 days' regeneration to establish definitely this enlargement as the stomach. From Table II it can be seen that the enlargement was present in this region in each case after 80 days' regeneration.

Traces of a muscle layer appeared in the anterior portion of the alimentary canal after 80 days' regeneration. Circular muscle fibres were distributed in the mesenchyme layer close to the outer epithelium. At this stage the mesenchyme cells between the muscle fibres and the inner epithelium were formed into the inner connective tissue as found in the normal gut. The narrow outer connective tissue layer, normally present between the muscle layer and the outer epithelium was not clearly distinguishable in the regenerating alimentary canal at this stage. Large oval “wandering” cells which are intensely eosinophilie occurred in the inner connective tissue layer and can be found in the mesentery among the mesenchyme cells. The latter are continuous with the inner connective tissue layer of the regenerating alimentary canal. The “wandering” cells resemble the oval and spheruliferous corpuscles described from the body wall and mesentery of S. chloronotus by Sivickis and Domantay (1928). They are present also in the connective tissue layer of the body wall of S. mollis. It seems possible that these cells are the carriers of nutritive reserves from the body wall via the mesentery to supply the proliferating cells which form the regenerating alimentary canal.

A study of the relative concentration of mitotic figures was made in an effort to determine the regions of greatest proliferation. Preparations showing mitotic figures were not obtained during early stages with the developing alimentary canal present as a solid cord of cells. After 40 days' regeneration, in regions which had developed a lumen, there was a great concentration of nuclei occurring around the lumen, and distinct mitotic figures were found as these cells proliferated to form the inner epithelium. The few observed in the inner connective tissue layer may be partly due to the much greater dispersal of nuclei in the matrix of the connective tissue. (Fig. 7.) In addition, although the connective tissue increases in total volume with enlargement of the alimentary canal, its relative thickness decreases with enlargement of the lumen in the alimentary canal, and with the differentiation of more cells to form the inner epithelium. Increase in the diameter of the alimentary canal necessitates some increase in the outer epithelial cells, to contain it, but division occurs here only in the plane parallel to the surface, and is much less prominent than that of the inner epithelium.

The alimentary canal contains all the normal histological layers after 110 days' regeneration. Before 145 days' regeneration, the layers differ in the poorer development of the muscle layer (especially of the longitudinal muscle fibres), and in the presence of a narrower outer connective tissue than that found in normal animals.