In any case our knowledge of the nature and origin of nervous plexuses is far too imperfect to found upon their characters such conclusions as those of Davidoff.
Gegenbaur, in his paper above quoted, further urges against Thacker and Mivart's views the fact that there is no proof that the fin of Polyodon is a primitive type; and also suggests that the epithelial line which I have found connecting the embryonic pelvic and pectoral fins in Torpedo may be a rudiment indicating a migration backwards of the pelvic fin.
With reference to the development of the pectoral fin in the Teleostei there are some observations of 'Swirski[492], 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.
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The observations which I have to lay before the Society were made with the object of determining how far the development of the skeleton of the limbs throws light on the points on which the anatomists whose opinions have just been quoted are at variance.
They were made, in the first instance, to complete a chapter in my work on comparative embryology; and, partly owing to the press of other engagements, but still more to the difficulty of procuring material, my observations are confined to the two British species of the genus Scyllium, viz. Sc. stellare and Sc. canicula; yet I venture to believe that the results at which I have arrived are not wholly without interest.
Before dealing with the development of the skeleton of the fin, it will be convenient to describe with great brevity the structure of the pectoral and pelvic fins of the adult. The pectoral fins consist of broad plates inserted horizontally on the sides of the body; so that in each there may be distinguished a dorsal and a ventral surface, and an anterior and a posterior border. Their shape may best be gathered from the woodcut (fig. 1); and it is to be especially noted that the narrowest part of the fin is the base, where it is[TN16] attached to the side of the body. The cartilaginous skeleton only occupies a small zone at the base of the fin, the remainder being formed of a fringe supported by radiately arranged horny fibres[493].
Fig. 1.
