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Show 500 PROF. HOWES AND MR. A. M. DAVIES ON THE [Dec. 4, teristic of the syndesmosis. Apart, however, from this important structural difference between the two things, there are certain other considerations which point to the conclusion that the syndesmodial pad is not, as might appear, a late infolding of the investing membrane. In many instances, especially in its phalangeal condition, it enters into that which is, at best, a loose connection with the same at all points (p.s.h., Hyla arborea, fig. 1), and there are developed true synovial surfaces at its points of contact with the penultimate and terminal phalanges. All this being so, the question next in need of solution is that of the primary origin of the syndesmodial p a d- Is it, as its histological structure in the larvae of Hyla and Rana would suggest, related to the phalanges 1-or is it, as its condition in Proteus would seem to indicate, a derivative of the investing tunic ? The base of the terminal phalanx is, in some Anura, greatly enlarged and swollen, giving rise (ex. Hylodes) to an immense tubercle which underlies the distal extremity of the penultimate one. In certain others the opposite ends of the phalanges are severally expanded, in a somewhat similar fashion ; and, when first our attention was drawn to the subject, it appeared not unlikely that the supernumerary phalanx might represent the dismembered tubercle of either the penultimate or terminal one-more probably that of the latter. The coexistence of the two things in Nototrema and other Hylids is, in itself, fatal to this supposition ; while it shows the structure in question to be of independent origin. Choice seemed then to lie between two alternatives ; it appeared to be either an intercalary structure of comparatively late origin, or a sesamoid. W e have already given reasons (supra) for doubting the former hypothesis. Our chief reason for supposing it to be a sesamoid lay in the assumption of a similarity of relationship with the proximal end of the terminal phalanx to that so common among the higher Vertebrata. This, however, is inconstant, for in many instances (ex. Rhacophorus eques, fig. 2) the structure is disposed lineally with the penultimate phalanx. Appeal to the details of muscular insertion proved fruitless, as the tendons become, for the most part, merged into the investing tissues proximally to the structure under consideration. The youngest stages examined by us were those of Rana temporaria and Hyla arborea. In the former, at a period at which the tail is almost absorbed, the condition of the joints recalls that of Proteus ; this is especially the case with the most distal joint (sy., sy.', fig. 10), which is strictly similar to that of the Urodele, except for the possession of a triangular projection from one side of the outer m e m brane. A similar condition is met with in Hyla arborea. Examination of our youngest specimen of this species shows, however, that, at a stage at which the parts in general are first becoming differentiated (fig. 9), the syndesmodial pad is centrally histologically identical, to minute detail, with the adjacent incipient phalanges ; while, peripherally, it graduates off into the surrounding indifferent tissue. Thus it is seen that the phalanges and syndesmoses are, together with their investing sheath, differentiations of a continuous |
