OCR Text |
Show 198 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, On the other hand, the latter is much like that of a Lamprey, if we leave the ossifications of the Menobranchus skull, and the accessory cartilages of the Petromyzon, out of consideration. And this fact, taken together with the curious resemblances in development between the Lampreys and the Amphibia (which are much closer than those between any of the higher Fishes and the Amphibia) *, suggest to m y mind the supposition that, in the series of modifications by which the Marsipobranch type has been converted into that of the higher fishes, the most important terms must have been forms intermediate in character between the Dipnoi and the Marsipobranchs. The skeleton of such a fish as Ceratodus, if it had a Menobranchus'-like chondrocranium, would approach that of the Lampreys more than that of any fish known at present; and it is not difficult to imagine the steps by which such a fish might be built up upon the " lines " of a Lamprey. The bearing of the structure of the chondrocranium in Menobranchus, the larval Triton and Siredon upon the theory of the skull is obvious. It is plain that three morphologically distinct elements enter into the composition of the cranium in these animals :- 1. The parachordal elements or "investing masses" of Rathke, which stand in the same relation to the notochord, in the skull, as the formative tissue out of which the bodies of the vertebrae are developed, in the spinal column. 2. The pleural elements or visceral arches, which are divisible into trabecular, mandibular, hyoidean, and branchial. 3. The paraneural elements or capsules of some of the organs of the higher senses. The brain-case is a complex structure, formed by the coalescence of elements belonging to all three classes; the face, by the metamorphosis of visceral arches only. The occipital portion of the chondrocranium, which lies behind the auditory capsules, and which, by its ossification, gives rise to the proper exoccipitals in Amphibia, and, in addition, to the basioccipital and supraoccipital in Osseous Fishes and the higher Vertebrata, appears in all cases to result from the metamorphosis and ossification of parachordal elements. O n the other hand, that portion of the chondrocranium which lies in front of the auditory capsules is, in the Amphibia, formed by the coalescence and metamorphosis of the trabecular. It is by their vertical growth that the relatively high lateral walls of the cranium of the Frog are formed; it is by their coalescence in the ventral median line that the pituitary space becomes completely floored with cartilage in the same animal; and it is by the outgrowth of alary processes from the coalesced internasal, or mesethmoidal, portions of the trabeculae, that the roof and floor of the Frog's nasal capsules are produced. Menobranchus is exceptional among the Amphibia in presenting no ossification of the substance of the trabeculae in front of the pro-otic. * Unfortunately we know nothing of the development of the Dipnoi. |