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Show 1888.] MORPHOLOGY OF SUPERNUMERARY PHALANGES. 497 it attains an adequate development as such, disposed ventrally (cf. freycineti, p.s, Plate XXIV. fig. 3), and there is a constancy of relationship between it and the adjacent head of the terminal phalanx(/..£, of fig. 3), the two generally coming to underlie, more or less completely, the distal epiphysis of the penultimate phalanx. This is, moreover, generally enlarged and invested in a fold of skin which projects freely forwards in the manner of a prominent lip, the whole giving to the an-tero- dorsal extremity of the digit a very characteristic aspect, to which Boulenger's fig. 1 does full justice. On comparison with Nototrema (fig. 7, * ) , in which the supernumerary phalanx attains but a small development, it is seen that this fold (*, fig. 7) is wholly related to the enlargement in question 1 ; and it will be found, in all cases, that the parts are so disponed as to allow of an upward rotation of the terminal phalanx. When the latter is fully displaced its long axis is seen to lie at right angles to that of the penultimate phalanx. It will be found on manipulation that the degree of ventral displacement of the supernumerary phalanx is here proportionate to that of the upward rotation of the terminal one, and that when the extremity of the digit is in contact with an applied surface, these two phalanges lie in the same plane, the former receiving, together with the base of the latter, the more direct thrust under the weight of the falling body. Such an arrangement would manifestly result in a distinct functional advantage, especially in the platydactyle forms, and the terminal phalanx would be the better able to support, undisturbed, the adhesive integument. When examined microscopically, the supernumerary phalanx is seen, in its fully differentiated condition, to consist in most cases of true hyaline cartilage (ex. Hyla arborea, fig. 1, and Rhacophorus, fig. 2 ) , differing in no respect from that forming the epiphysial extremities of the adjacent phalanges. It remains in this condition long after the other phalanges, including the terminal one, have become ossified (cf. Rhacophorus). It invariably ossifies quite late; and among the large series of specimens examined we have met with it in the bony condition only in Hyla freycineti and Rhacophorus maximus. In the former instance it is seen (fig. 3) to be replaced in a true endostosis. It might appear from the foregoing that its first development takes place subsequently to that of the other phalanges, and that its ossification sets in at a period relatively proportionate to that of the same. Examination of the tadpole of Hyla arborea (fig. 1) shows that this is not the case, for it is there as fully differentiated as with the adult, and that at a stage in which the periosteal growth of the adjacent elements is dawning. Nor must it be imagined that its conversion into bone is in any way determined by its relative size, for in Rhacophorus eques (fig. 2, p.s.) it is, while still unossified, relatively larger than in any species with which we have dealt. On passing from the ossified type to that of the other extreme of the series (to those forms, that is, in the adults of which anything 1 In the larva the conditions are otherwise, cf. Hyla, fig. 1. |
