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Show 146 PROF. G. B. HOWES AND w. R I D E W O O D O N [Mar. 0, whether this enlargement may or may not represent a coalesced 5th tarsal (as, indeed, Gegenbaur was inclined to believe, pp. 64-65). W e have failed to detect any trace of segmentation of the cartilage at this point. In the Hylidce, however, a second ligament is present, which passes (fig. 19) from the postaxial wall of the capsule of the tarso-metatarsal joint, upwards and inwards, for attachment to the epiphysial cartilage of the astragalus and calcaneus. This ligament carries at its outer end a wedge-shaped cartilage, and that we at first took to represent the fifth tarsal. Upon reconsideration, however, we are disposed to regard it as an ordinary intra-articular cartilage of none but physiological significance (for further details see p. 168). Reflection upon these facts has led us to the belief that while the ligament which passes between the third tarsal and the head of the 5th metatarsal is the degraded representative of the 4th and 5th tarsalia, the cartilaginous nodule occasionally carried by it is exclusively a vestige of the 4th one. Gegenbaur records the presence of this ligament in Mana esculenta, in addition to the other genera named, and we can confirm his statement. In R. temporaria, however, we find no trace of it in the adult, and but a feeble one in the larva. This fact, while lending additional support to Gegenbaur's original deduction, shows how completely the vestige is, as it were, disappearing under our eyes. d. Tarsalia 2 and 3 (Os cuboideum).-We have already stated that three tarsal elements may exist; on the other hand, the outermost two of these may not unfrequently be replaced in a long bone (cuboideum), represented in its typical condition at 2 3 in figs. 17, 24, 27, 29, 31, & 33. It will be observed iu all these cases that this structure overlies, more or less completely, the metatarsalia of the 2nd and 3rd digits ; comparison of the same with those forms in which the three tarsalia are found (Bombinator or Alytes, figs. 8 & 10) leaves little doubt but that the bone represents a confluence of the tarsalia of the 2nd and 3rd digits, as asserted by all recent authorities. In some cases, however, examination of the adult limb would appear to render this questionable. For example : in Hyla c&rulea (fig. 19), in which two tarsalia are alone present, the larger one is little, if at all, related to the 2nd metatarsal-the head of which is in apposition with the second and smaller element. Comparison of this tarsus with that possessed of the three tarsalia (figs. 8 & 10) renders it hard, indeed, to say which two elements are represented in the Tree-Frog. Born has already experienced this difficulty in dealing (6, p. 443) with the adults of Phryne vulgaris and Bufo variabilis, and he admits his inability to settle the question, for want of embryological material. On turning to the larva of Hyla (fig. 21), we find that the outer of the two elements has (unlike that of the adult) more than twice the bulk of its fellow, while it agrees in every detail with the cuboideum in its most typical form (cf. Xenophrys, fig. 17). Examination of this element side by side with that of the adult limb (fi°\ 19) shows that its growth is arrested early in development: it shows, |
