The walls both of the vesicle and stalk are formed of a fairly columnar epithelium. The vesicle communicates in front by a narrow passage with the neural canal, and behind is continued into two horns corresponding with the two caudal swellings previously spoken of (p. 55). Where the canal is continued into these two horns, its walls lose their distinctness of outline, and become continuous with the adjacent mesoblast.
In the succeeding stages, as the tail grows longer and longer, the postanal section of the alimentary tract grows with it, without however undergoing alteration in any of its essential characters. At the period of the maximum development, it has a length of about 1⁄3 of that of the whole alimentary tract.
Its features at a stage shortly before the external gills have become prominent are illustrated by a series of transverse sections through the tail ([fig. 424]). The four sections have been selected for illustration out of a fairly-complete series of about one hundred and twenty.
Posteriorly (A) there is present a terminal vesicle (alv) .25 mm. in diameter, which communicates dorsally by a narrow opening with the neural canal (nc); to this is attached a stalk in the form of a tube, also lined by columnar epithelium, and extending through about thirty sections (B al). Its average diameter is about .084 mm., and its walls are very thick. Overlying its front end is the subnotochordal rod (x), but this does not extend as far back as the terminal vesicle.
The thick-walled stalk of the vesicle is connected with the cloacal section of the alimentary tract by a very narrow thin-walled tube (C al). This for the most part has a fairly uniform calibre, and a diameter of not more than .035 mm. Its walls are formed of flattened epithelial cells. At a point not far from the cloaca it becomes smaller, and its diameter falls to .03 mm. In front of this point it rapidly dilates again, and, after becoming fairly wide, opens on the dorsal side of the cloacal section of the alimentary canal just behind the anus (D al).
Fig. 424. Four sections through the postanal part of the tail of an embryo of the same age as fig. 28 F.
A. is the posterior section.
nc. neural canal; al. postanal gut; alv. caudal vesicle of postanal gut; x. subnotochordal rod; mp. muscle-plate; ch. notochord; cl.al. cloaca; ao. aorta; v.cau. caudal vein.
Very shortly after the stage to which the above figures belong, at a point a little behind the anus, where the postanal section of the canal was thinnest in the previous stage, it becomes solid, and a rupture here occurs in it at a slightly later period.
The atrophy of this part of the alimentary tract having once commenced proceeds rapidly. The posterior part first becomes reduced to a small rudiment near the end of the tail. There is no longer a terminal vesicle, nor a neurenteric canal. The portion of the postanal section of the alimentary tract, just behind the cloaca, is for a short time represented by a small rudiment of the dilated part which at an earlier period opened into the cloaca.