| OCR Text |
Show 1874.] PROF. T. H. HUXLEY ON MENOBRANCHUS. 199 In all the other genera, so far as I am aware, these structures are ossified to a greater or less extent in front of the exit of the optic nerves. In Proteus, the ossification is internasal only; in Siredon and Menopoma, it is interorbital only; in Siren, there are two extensive interorbital ossifications, which send median prolongations into the internasal septum, and thus afford a transition to the fully developed sphenethmoid (or "os en ceinture" of Cuvier) of the Frogs. I see no reason for doubting the homology of the paired interorbital ossifications of Siredon with the orbito-sphenoids of the higher Vertebrata; in which case, that portion of the basis of the skull which arises out of the coalescence of these parts of the trabeculae, and is ossified into one mass with them, in the Frogs, must represent the presphenoid; and if this be so, the floor and side-walls of the skull, between the interorbital ossification and the pro-otic bones, must answer to the basisphenoid and the alisphenoid; while that which lies in front of the interorbital ossification must correspond with the median and lateral ethmoids of the higher Vertebrates. In the Amphibia all these parts are formed by the gradual extension and subsequent metamorphosis of the trabeculae. All the steps of gradual enlargement, apparent outgrowth, and metamorphosis of these primitively rod-like cartilages can be followed; and no part of the chondrocranium in these regions is formed independently of them. This is all I intend to convey by the expression that the sphenoidal and ethmoidal regions of the skull are products of the growth and metamorphosis of the trabeculae. If the questions be raised, Have the trabeculae, when once formed, a quasi-independence 1 and do they grow into the adjacent tissues, as a tree pushes its roots into the soil ? Or does not their extension and apparent growth arise rather from a chondrification of the preexisting tissue in the immediate neighbourhood of the trabecular cartilage ? it seems to m e that no definite answer can be given to them. In the larval Triton and Siredon, at the stage of development described above, for example, there is no complete cartilage either at the sides of the notochord, behind the trabeculae, or in the ethmoidal region, in front of the trabeculae. And it would seem that the cartilage which eventually exists in both these regions, arises in the same way-namely, by gradual chondrification of the tissue, beginning in that part which is in contact with the trabecula, and extending backwards, or forwards, as the case m a y be. And it may be said that if the apparent growth of the trabecula into the parachordal region is not to be described as a backward growth of the trabecula, so neither is the alisphenoidal, or orbito-sphenoidal, cartilage in the side-wall of the skull to be described as an upward growth of the trabecula; and this view would receive support from any cases in which the orbito-sphenoids, or alisphenoids, take their origin by independent development in the side-walls of the skull. The same difficulty arises when we attempt to determine the nature of the cartilaginous walls of the nasal chambers. To all appearance these, in all Amphibia which possess them, grow out of the coalesced trabeculae. But if it be said that they are independent |
