Haemal System.

The ventral intestinal vessel commences after 30 days' regeneration as an extension of the mesenchyme cells with covering epithelium from along the ventral side of the alimentary canal. At intervals the outer epithelium fuses in the mid-line between the gut and the developing vessel. At these points the mesenchyme in the vessel is isolated from that of the gut and separation of the ventral vessel commences. The region in which the two mesenchyme masses remain in continuity marks the origin of the small connecting vessels between the gut and the ventral vessel. This separation was first observed after 33 days' regeneration in Specimen 67, where it was rest rioted to the posterior two-thirds of the gut forming along the dorsal mesentery. It was not then continuous with the original haemal vessel remnants along the oesophagus, and did not extend up the lateral mesentery or further posteriorly. Other specimens examined up to 60 days showed no further development, some showing no separation of the vessel, even from alimentary canals of greater thickness. After 65 days' regeneration the ventral vessel was found continuous as a solid cord of cells connected with the original remnant along the oesphagus, but separation from the posterior portion of the alimentary canal had not occurred. Specimen 98, after 80 days' regeneration, showed a continuous ventral vessel widest anteriorly at its point of junction with the original vessel remnant, and continuing conspicuously but narrowing until half-way along the ventral mesentery, where it ceased.

The dorsal intestinal vessel develops close to the junction of the mesentery and gut. It is formed later than the ventral vessel but follows the same stages in its formation. After 80 days regeneration it could be traced from the connection with the original vessel at the oesophogeal remnant to the anterior portion of the gut along the ventral mesentery. At this stage it was less prominent than, and