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Show 1888.] MORPHOLOGY OF SUPERNUMERARY PHALANGES. 509 to regard the supernumerary elements as primarily intercalary, and all probability derivative of the inter-phalanges (inter-articular syndesmoses-the " Zwischenscheibene " of Henke and Reyher, 6). W e accordingly accept, so far as it might bear upon primary differentiation of the parts, Leboucq's declaration (13. p. 532) that " aile Phalangen [in the Cetacea] wie gross ihre Zahl sein moge, denselben morphologischen Wert haben." The condition of the limbs in the living Sirenia referred to (ante, p. 507) suggests that the numerical increase of the phalanges may have been associated with the loss of the ungues, and it is interesting to reflect here that elongation by regular segmentation of the cartilaginous rays of the paired fins of the Batoidei would, as compared with those of the Selachii, appear to have been somewhat similarly associated with the suppression of the horny fin-rays. The condition of the parts in Squatina and Zygcena would seem to be transitional in this respect. W e put this forward as a mere suggestion, deduced by argument from structural analogy. It would be exceedingly instructive, in the light of the preceding, to ascertain if the syndesmosis-like pads of the toothed whales pass through a synovial stage during development. Henke and Reyher's observations already cited (p. 502) show that in their ' Zwischenschei-ben' we have to deal with a derivative of the syndesmosis. Our attention was early arrested by the general similarity between the proximal syndesmoses in the Hyiids and Ranids and the knee-joint in the higher Vertebrata. Comparison of the parts in the latter with those of the former as represented by Nototrema (p. 502) reveals a striking uniformity between the two ; and, should it be found that the semilunar cartilages are, with their ligaments, differentiations of a syndesmosis, a complete reconsideration of the morphological value of the former will be imperative. W e have examined some Mammalian embryos in respect to this, but we withhold, for the present, further comment thereon. It is pertinent to recall here Parker's assertion (15. p. 487) that in Aves (Apteryx) the rudiment of the mesotarsal semilunar pad bears in its centre " a rounded nodule of hyaline cartilage," which he takes to be " the representative of the centrale tarsi, an element not hitherto recognized in birds." VI. CONCLUSIONS. 1. That the supernumerary phalanx of the Anura is a true phalanx, and, at the same time, structurally identical with the inter-phalangeal syndesmosis of these and the other Amphibia, all transitions between tbe two being represented in adults of the living forms. 2. That the syndesmoses and phalanges are differentiations of a common blastema. 3. That the supernumerary phalanx of the Anura is probably PROC. ZOOL. Soc-1888, No. XXXV. 35 |
