| OCR Text |
Show 498 PROF. HOWES AND MR. A. M. DAVIES ON THE [Dec. 4, like a conspicuous structure is interposed between the terminal and penultimate phalanges) we meet (ex. the common Frog, Plate X X V. fig. 16) with a pad of loose fibrous tissue, which is closely bound down to the applied epiphysial surfaces, such as would appear to have nothing to do with the undoubted phalanx under consideration. In Nototrema, however (fig. 7), the pad, while never truly hyaline, has all the fundamental relations described for the phalanx in the culminating term of the series ; and in Hyla peronii (fig. 13), while histologically identical with that of Nototrema, it has the proportions, detailed shape, and relationships seen in H. arborea. Hyla ccerulea bridges over the gap histologically between Nototrema and Hyla arborea ; and when to this it is added that, in those Ranidce the adults of which bear the supernumerary phalanx, the same early becomes hyaline as in Hyla arborea, it is clear that all stages between the two extremes afore described are forthcoming. Minor modifications are met with, but these may, together with the consideration of lesser detail, preferably be dealt with later. Seeing that, upon a purely structural analysis, the fibrous pad of the Commcn Frog must be looked upon as the homologue of the skeletal supernumerary phalanx of the higher Ranidce and Hylida?, we are next led to inquire what, if any, structural community the two may possess in the embryo. On examination of the larvse of Hyla arborea and Rana temporaria, at a stage at which the hind limbs are becoming differentiated, it is found that the places of the phalanx of the former, and of the pad in the latter, are alike occupied by a fibrous mass which is largely cellular and loosely interposed between the applied epiphysial extremities. It is well known that, in most Urodeles, there are interposed between the corresponding parts of the limb-skeleton fibrous masses which take the place of the synovial capsules of the higher Vertebrata ; and these are found, on examination, to be indistinguishable from those above described. They have long been termed by Hyrtl (8) " syndesmoses." The foregoing is not all. Hyrtl, in describing the manus of Salamandra maculosa, writes (p. 61): "phalanges inter se, et cum ossibus metacarpi, textu fibroso conjunguntur." W e find that these syndosmoses are structurally identical with the supernumerary phalanx of the Anura in its least modified condition (cf. figs. 11, 12), and, seeing that in many Anura similar pads are interposed between the proximal phalanges themselves, it follows that the structures in question in them are not in any sense to be regarded as peculiar to the terminal segments of the digits, and that the supernumerary phalanx would appear to be a specialized counterpart of the inter-phalangeal syndesmosis. In other words, may not the community of structure between the developing supernumerary phalanx and the syndesmosial pad be indicative of a community of origin? Should this be the case, there would open up a new and promising departure for the reconsideration of the questions of origin and morphology of supernumerary phalanges in general. To these we shall return. |
