Gegenbaur derives the Elasmobranch pectoral fin from a form which he calls the archipterygium, nearly like that of Ceratodus, with a median axis and two rows of rays—but holds that in addition to the rays attached to the median axis, which are alone found in Ceratodus, there were other rays directly articulated to the shoulder-girdle. He considers that in the Elasmobranch fin the majority of the lateral rays on the posterior (or median according to his view of the position of the limb) side have become aborted, and that the central axis is represented by the metapterygium; while the pro- and mesopterygium and their rays are, he believes, derived from those rays of the archipterygium which originally articulated directly with the shoulder-girdle.
This view appears to me to be absolutely negatived by the facts of development of the pectoral fin in Scyllium—not so much because the pectoral fin in this form is necessarily to be regarded as primitive, but because what Gegenbaur holds to be the primitive axis of the biserial fin is demonstrated to be really the base, and it is only in the adult that it is conceivable that a second set of lateral rays could have existed on the posterior side of the metapterygium. If Gegenbaur's view were correct, we should expect to find in the embryo, if anywhere, traces of the second set of lateral rays; but the fact is that, as may easily be seen by an inspection of figs. 6 and 7, such a second set of lateral rays could not possibly have existed in a type of fin like that found in the embryo. With this view of Gegenbaur's it appears to me that the theory held by this anatomist to the effect that the limbs are modified gill-arches also falls, in that his method of deriving the limbs from gill-arches ceases to be admissible, while it is not easy to see how a limb, formed on the type of the embryonic limb of Elasmobranchii, could be derived from a gill-arch with its branchial rays.
Gegenbaur's older view, that the Elasmobranch fin retains a primitive uniserial type, appears to me to be nearer the truth than his more recent view on this subject; though I hold the fundamental point established by the development of these parts in Scyllium to be that the posterior border of the adult Elasmobranch pectoral fin is the primitive base-line,i.e.line of attachment of the fin to the side of the body.
Huxley holds that the mesopterygium is the proximal piece of the axial skeleton of the limb of Ceratodus, and derives the Elasmobranch fin from that of Ceratodus by the shortening of its axis and the coalescence of some of its elements. The entirely secondary character of the mesopterygium, and its total absence in the young embryo Scyllium, appear to me as conclusive against Huxley's view as the character of the embryonic fin is against that of Gegenbaur; and I should be much more inclined to hold that the fin of Ceratodus has been derived from a fin like that of the Elasmobranchii by a series of steps similar to those which Huxley supposes to have led to the establishment of the Elasmobranch fin, but in exactly the reverse order.
There is one statement of Davidoff's which I cannot allow to pass without challenge. In comparing the skeletons of the paired and unpaired fins he is anxious to prove that the former are independent of the axial skeleton in their origin and that the latter have been segmented from the axial skeleton, and thus to shew that an homology between the two is impossible. In support of his view he states[495] that he has satisfied himself, from embryos of Acanthias and Scyllium, that the rays of the unpaired fins are undoubtedly products of the segmentation of the dorsal and ventral spinous processes.
This statement is wholly unintelligible to me. From my examination of the development of the first dorsal and the anal fins of Scyllium I find that their rays develop at a considerable distance from, and quite independently of, the neural and hæmal arches, and that they are at an early stage of development distinctly in a more advanced state of histological differentiation than the neural and hæmal arches of the same region. I have also found exactly the same in the embryos of Lepidosteus.
I have, in fact, no doubt that the skeleton of both the paired and the unpaired fins of Elasmobranchii and Lepidosteus is in its development independent of the axial skeleton. The phylogenetic mode of origin of the skeleton both of the paired and of the unpaired fins cannot, however, be made out without further investigation.
EXPLANATION OF PLATE 33.[496]
Fig. 1. Transverse section through the pelvic fin of an embryo of Scyllium belonging to stage P1, magnified 50 diameters. bp. basipterygium. br. fin ray. m. muscle. hf. horny fibres supporting the peripheral part of the fin.
Fig. 2. Pelvic fin of a very young female embryo of Scyllium stellare, magnified 16 diameters. bp. basipterygium. pu. pubic process of pelvic girdle (cut across below). il. iliac process of pelvic girdle. fo. foramen.