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Show 1888.] MORPHOLOGY OF SUPERNUMERARY PHALANGES. 499 III. HISTOLOGICAL AND DEVELOPMENTAL. Dixey, in a paper on the ossification of the terminal phalanges (3), incidentally describes and figures the syndesmosis in Proteus. He deals with that interposed between the penultimate and terminal phalanges-with that, that is to say, which would appear to represent the supernumerary phalanx of the Anura. He writes (p. 68) : " another instance of arrested development in the digit of Proteus is afforded by the inter-phalangeal joint . . . . the cartilage, with a slight alteration in the size and relative number of its cells, is seen to be quite continuous between the heads of the two phalanges, nor does it exhibit the least sign of an articular cavity." He believes his specimen to have been "fully adult " (p. 70) ; but in this he was mistaken, for in the larger of our specimens of the same (cf. p. 504) ossification had proceeded much further than in his. Microscopic examination of this syndesmosis with its associated parts in Proteus reveals, under the treatment which we have adopted (p. 496), the following facts. The matrix of the epiphysial cartilages of the phalanges stains uniformly and feebly, while that of the less resistant syndesmosis takes the dye much more readily, becoming thereby sharply differentiated (cf" Plate X X V . fig. 12). The latter shows traces of a fibrillar structure, but it is for the most part homogeneous. The cells which are present are well defined, and their peculiarities in structure, disposition, and size are common to both epiphysis and syndesmosis. Each is irregular in contour, and carries a large nucleus, while it is seen to lie within a spacious lacuna, the boundary of which is smooth and well defined ; and such differences as are met with between the corpuscles of the epiphyses and of the syndesmosis are seen to be entirely due to pressure under apposition of the first named. The cells of the syndesmosis are more numerous and more closely aggregated than those of the hyaline epiphyses, and the intensity of colour of the former under the action of reagents is, to a large extent, due to this crowding. The details of histological structure here described hold good, with but slight modifications, for all conditions in which the supernumerary phalanx and its homologue remain non-hyaline (cf.Proteus, fig. 12, and Hyla peronii, fig. 14). Its cells are cartilage corpuscles, and the tissue to which they give rise is, in its most elementary form, a nascent cartilage. The whole digital skeleton is invested in a continuous and well-differentiated fibrous tunic, and in Proteus this is, at any rate ventrally, incompletely marked off from the syndesmodial pad. There lie buried up in the former at this point (sh., fig. 12) cells which closely resemble those of the syndesmosis in size and structure ; the question, therefore, naturally arises as to whether some of these might not have migrated into the inter-phalangeal region, there to give rise to the pad in question, or that that might conceivably have been an ingrowth of the tunic itself. It will be seen, however, that the corpuscles of the tunic lie buried in a coarsely fibrous matrix, in which there is a total absence of the lacunae so charac- |