| OCR Text |
Show 150 PROF. G. B. HOWES AND W. RIDEWOOD ON [Mar. 6, adults of certain species, may or may not arise by confluence or suppression of individual segments (cf. fig. 8 ) ; future research must decide this question. There remain two most important questions. 1, If the segmented condition be the more primitive, may not the whole pre-hallux represent a shortened-up digit ? and 2, May not this supposed sixth digit represent a lost ray of the ichtbyopterygium ? In Nannophrys, Phyllomedusa (fig. 19 a), Rhombophryne, and especially in some species of Hyla, the pre-hallux has quite a digiti-form aspect, comparing, at first sight, very favourably with the reduced pollex of the fore limb, the homology of which with the other digits nobody disputes (cf. Plate VIII. figs. 19 a & 20). That the pre-hallux may, in a sense, resemble the pollex is clear from the preceding; but it must not be forgotten that this resemblance is most marked in the specialized Tree-Frogs. Its segments1 are, in the embryo, more digitiform than in the adult; and taking all facts into consideration, we incline to a belief in a preponderance of the resemblances over the differences between this structure and a normal digit. The second difficulty is not easily to be met. That the pre-hallux takes on certain of the essential relationships of a digit is beyond dispute. That it really represents one is another question. The tarsal of the pre-hallux is, in most cases, in definite articulation with the head of the naviculare (cf. Pelodytes, fig. 13). In many forms its second segment is the largest (ex. fig. 27); when this is the case, that may be disposed parallel with the metatarsals, in a manner strikingly suggestive of a serial homology (cf. Xenopus, fig. 3, and Pseudis, fig. 27). In Xenophrys, Hyla (figs. 17 and 19), and other genera, this supposed metatarsal sends forwards (backwards in situ) a small retral lobe (*) ; in Ceratophrys (fig. 24) this attains a considerable development, and in individuals of .this genus it may exceed in size the body of the segment from which it takes its origin. Meckel (29) and Cuvier (15) held, and Born (3), Kehrer (24), and others still hold the pre-ballux to be a sixth digit. Leydig (25), attacking it from a totally different standpoint, argues to the contrary. Gegenbaur at first took a similar view, holding (18) it to be a secondary structure peculiar to the Anura, and his words are echoed by Hofmann(21); subsequently however to the publication of Born's paper, Gegenbaur accepted that author's position, confessing to the same in his well-known text-book. Finally, B om asserts, in his latest paper (6, p. 61) " Ubrigens bin ich jetzt geneigt in der starken Variabilitiit der Gebilde der sechsten Zehe . . . nicht bloss mehr eine Eigenthtimlichkeit zu sehen, die derselbem als rudimentarem Organ anhaftet, sondern ich sehe in der haufigen Verschmelzuug ein Bestreben ein immer festeres Skelettstuck als 1 Van Deen records (34) an instance in which, in Rana esculenta, these lay in a line with the hallux and supported the web. His specimen was, unfortunately, a monstrous one, with four hind legs. |
