The portion of the intestine behind the vitelline duct is, as in all the Vertebrata, at first straight. In Elasmobranchii the lumen of the part of the intestine in which a spiral valve is present in the adult, very early acquires a more or less semilunar form by the appearance of a fold which winds in a long spiral. In Lepidosteus there is a fold similar in every respect (Plate 38, fig. 53, sp.v.), forming an open spiral round the intestine. This fold is the first indication of the spiral valve, but it is relatively very much later in its appearance than in Elasmobranchii, not being formed till about three weeks after hatching. It is, moreover, in correlation with the small extent of the spiral valve of the adult, confined to a much smaller portion of the intestine than in Elasmobranchii, although owing to the relative straightness of the anterior part of the intestine it is proportionately longer in the embryo than in the adult.
The similarity of the embryonic spiral valve of Lepidosteus to that of Elasmobranchii shews that Stannius' hesitation in accepting Müller's discovery of the spiral valve in Lepidosteus is not justified.
J. Müller (Bau u. Entwick. d. Myxinoiden) holds that the so-called bursa entiana of Elasmobranchii (i.e., the chamber placed between the part of the intestine with the spiral valve and the end of the pylorus) is the homologue of the more elongated portion of the small intestine which occupies a similar position in the Sturgeon. This portion of the small intestine is no doubt homologous with the still more elongated and coiled portion of the small intestine in Lepidosteus placed between the chamber into which the pyloric cæca, &c., open and the region of the spiral valve. The fact that the vitelline duct in the embryo Lepidosteus is placed close to the pyloric end of the stomach, and that the greater portion of the small intestine is derived from part of the alimentary canal behind this, shews that Müller is mistaken in attempting to homologise the bursa entiana of Elasmobranchii, which is placed in front of the vitelline duct, with the coiled part of the small intestine of the above forms. The latter is either derived from an elongation of the very short portion of the intestine between the vitelline duct and the primitive spiral valve, or more probably by the conversion of the anterior part of the intestine, originally provided with a spiral valve into a coiled small intestine not so provided.
We have already called attention to the peculiar mesentery present in the adult attaching the posterior straight part of the intestine to the ventral wall of the body. This mesentery, which together with the dorsal mesentery divides the hinder section of the body-cavity into two lateral compartments is, we believe, a persisting portion of the ventral mesentery which, as pointed out by one of us[550], is primitively present for the whole length of the body-cavity. The persistence of such a large section of it as that found in the adult Lepidosteus is, so far as we know, quite exceptional. This mesentery is shewn in section in the embryo in Plate 38, fig. 53 (v.mt.). The small vessel in it appears to be the remnant of the subintestinal vein.
[547] For a history of similar nuclei, vide Comp. Embryol., Vol. II., chapters III. and IV.
[548] Vide Comp. Embryol., Vol. II., pp. 50-63 [the original edition].
[549] Vide F. M. Balfour, “Monograph on Development of Elasmobranch Fishes,” p. 226 [This edition, No. X., p. [454]].
[550] Comparative Embryology, Vol. II. p. 514 [the original edition].
The Gill on the Hyoid Arch.
It is well known that Lepidosteus is provided with a gill on the hyoid arch, divided on each side into two parts. An excellent figure of this gill is given by Müller (No. 13, plate 5, fig. 6), who holds from a consideration of the vascular supply that the two parts of this gill represent respectively the hyoid gill and the mandibular gill (called by Müller pseudobranch). Müller's views on this subject have not usually been accepted, but it is the fashion to regard the whole of the gill as the hyoid gill divided into two parts. It appeared to us not improbable that embryology might throw some light on the history of this gill, and accordingly we kept a look out in our embryos for traces of gills on the hyoid and mandibular arches. The results we have arrived at are purely negative, but are not the less surprising for this fact. The hyomandibular cleft as shewn above, is never fully developed, and early undergoes a complete atrophy—a fact which is, on the whole, against Müller's view; but what astonished us most in connection with the gill in question is that we have been unable to find any trace of it even in the oldest larva whose head we have had (26 millims.), and at a period when the gills on the hinder arches have reached their full development.