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 secondary character of the mesopterygium, and its total absence in the embryo Scyllium, appears 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.
With reference to the development of the pectoral fin in the Teleostei there are some observations of ’Swirski (No. [488]) which unfortunately do not throw very much light upon the nature of the limb.
’Swirski finds that in the Pike the skeleton of the limb is formed of a plate of cartilage, continuous with the pectoral girdle; which soon becomes divided into a proximal and a distal portion. The former is subsequently segmented into five basal rays, and the latter into twelve parts, the number of which subsequently becomes reduced.
These investigations might be regarded as tending to shew that the basipterygium of Elasmobranchii is not represented in Teleostei, owing to the fin rays not having united into a continuous basal bar, but the observations are not sufficiently complete to admit of this conclusion being founded upon them with any certainty.
The cheiropterygium.
Observations on the early development of the pentadactyloid limbs of the higher Vertebrata are comparatively scanty.
The limbs arise as simple outgrowths of the sides of the body, formed both of epiblast and mesoblast. In the Amniota, at all events, they are processes of a special longitudinal ridge known as the Wolffian ridge. In the Amniota they also bear at their extremity a thickened cap of epiblast, which may be compared with the epiblastic fold at the apex of the Elasmobranch fin.
Both limbs have at first a precisely similar position, both being directed backwards and being parallel to the surface of the body.
In the Urodela (Götte) the ulnar and fibular sides are primitively dorsal, and the radial and tibial ventral: in Mammalia however Kölliker states that the radial and tibial edges are from the first anterior.
The exact changes of position undergone by the limbs in the course of development are not fully understood. To suit a terrestrial mode of life the flexures of the two limbs become gradually more and more opposite, till in Mammalia the corresponding joints of the two limbs are turned in completely opposite directions.