An account has already been given (p. [307]) of the posterior continuity of the neural and alimentary canals, and it was there stated that Kowalevsky was the discoverer of this peculiar arrangement. Since that account was published, Kowalevsky has given further details of his investigations on this point, and more especially describes the later history of the hindermost section of the alimentary tract. He says[303]:
The two germinal layers, epiblast and hypoblast, are continuous with each other at the border of the germinal disc. The primitive groove or furrow appears at the border of the germinal disc and is continued from the upper to the lower side. By the closing of the groove there is formed the medullary canal above, while the part of the groove on the under surface directed below is chiefly converted into the hind end of the alimentary tract. The connection of the two tubes in Acanthias persists till the formation of the anus, and the part of the nervous tube which lies under the chorda passes gradually upwards to the dorsal side of the chorda, and persists there for a long time in the form of a large thin-walled vesicle.
The last part of the description beginning at “The connection of” does not hold good for any of the genera which I have had an opportunity of investigating, as will appear from the sequel.
In a previous section[304] the history of the alimentary tract was completed up to stage G.
In stage H the point where the anus will (at a very much later period) appear, becomes marked out by the alimentary tract sending down a papilliform process towards the skin. This is shewn in Pl. 8, figs. H and I, an.
That part of the alimentary tract which is situated behind this point may, for convenience, be called the postanal section. During stage H the postanal section begins to develop a terminal dilatation or vesicle, connected with the remainder of the canal by a narrower stalk. The relation in diameter between the vesicle and the stalk may be gathered by a comparison of figs. 3a and 3b, Pl. 11. The diameter of the vesicle represented in section in Pl. 11, fig. 3, is 0.328 Mm.
The walls both of the vesicle and stalk are formed of a fairly columnar epithelium. The vesicle communicates in front by a narrow passage (Pl. 11, fig. 3a) with the neural canal, and behind is continued into two horns (Pl. 11, fig. 2, al.) corresponding with the two caudal swellings spoken of above (p. [288]). 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 up to K the tail grows longer and longer, and with it grows the postanal section of the alimentary tract, without however altering in any of its essential characters.
Its features at stage K are illustrated by an optical section of the tail of an embryo (Pl. 18, fig. 5) and by a series of transverse sections through the tail of another embryo in Pl. 18, figs. 6a, 6b, 6c, 6d. In the optical section there is seen a terminal vesicle (alv.) opening into the neural canal, and connected with the remainder of the alimentary tract. The terminal vesicle causes the end of the tail to be dilated, as is shewn in Pl. 8, fig. K. The length of the postanal section extending from the abdominal paired fins to the end of the tail (equal to rather less than one-third of the whole length of the embryo), may be gathered from the same figure.
The most accurate method of studying this part of the alimentary canal is by means of transverse sections. Four sections have been selected for illustration (Pl. 18, figs. 6a, 6b, 6c, and 6d) out of a fairly-complete series of about one hundred and twenty.