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Show 196 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, quite so instructive in its bearing on that of Menobranchus as is the skull of a Triton about the period at which it leaves the egg * (Plate X X X I . figs. 2, 2a), or that of a larval Axolotl (Plate X X X I . fig. 1). Here the notochord occupies the centre of the future basis cranii, terminating in front in a rounded apex. The parachordal tissue exhibits no trace of chondrification ; but it is very interesting to observe, on each side of it, the indication of an intermuscular septum, separating two myotomes, and thus indicating, so far, a segmentation of this region. The auditory capsules are spherical sacs, which lie, quite isolated, on each side of the notochord, at some distance from it, and are not yet chondrified. The trabeculse abut against the notochord posteriorly, but are wholly separate from any other structure. In front, they have begun to coalesce and to give rise to the broad internasal plate which is characteristic of the Salamandridea. On each side of them are seen the nasal sacs, with the minute posterior nares opening into the cavity of the mouth. Behind, and external to, each trabecula is a stout cartilaginous rod, which obviously represents the suspensorium ; but the dorsal end of this cartilage (m), though it lies close to the trabecula, has not yet coalesced with it, and the mandibular arch is therefore quite free. The external angle (o) corresponds in its relation to the auditory capsule with the part similarly marked in the Frog's tadpole and in Menobranchus. At (q) is the articular surface for Meckel's cartilage (Mck); but neither "orbital," "ascending," nor " palato-pterygoid " processes are as yet developed. The hyoidean arch (Hy, figs. 2, 2 a) is as distinct and independent as are the mandibular and trabecular arches ; it is an unjointed cartilage with a pointed dorsal end, which lies close to the auditory capsule. At its ventral extremity it coalesces with its fellow; while, behind, it is continuous with a median cartilage, which represents the basibranchials and ends in a long spatuliform style. From the sides of the median cartilage two ceratobranchials proceed, and are continued, the anterior into the first epibranchial, the posterior into the three other epibranchials. None of these parts are distinctly articulated, the future joints being, at most, faintly indicated. Sundry ossifications are visible in the fibrous tissue contiguous to the cartilages; thus the dentary (D) and splenial (Spl) pieces of the mandible, the squamosals (Sq), and the preemaxillee (Pmx) (already one bone) have made their appearance. The vomers (Fo), each of which bears two teeth, lie far apart, on the inner side of each nasal opening, and beneath the anterior end of the trabecula. Behind these are two dentigerous ossifications of the fibrous roof of the mouth, broad and rounded in front, but drawn out behind into a sort of tail, which is directed towards the suspensorium, though it does not reach the latter. These bones correspond with the anterior moieties of the palato-pterygoids of Menobran- * The observations on which the following statements respecting Triton rest, were made in 1858; but I did not publish them, as I could not then obtain the materials for completing the history of the development of the Triton's skull. Perhaps I shall be more fortunate this spring. |
