In Scyllium the development of both the pectoral and pelvic fins is very similar.
In both fins the skeleton in its earliest stage consists of a bar springing from the posterior side of the pectoral or pelvic girdle, and running backwards parallel to the long axis of the body. The outer side of this bar is continued into a plate which extends into the fin, and which becomes very early segmented into a series of parallel rays at right angles to the longitudinal bar.
In other words, the primitive skeleton of both the fins consists of a longitudinal bar running along the base of the fin, and giving off at right angles series of rays which pass into the fin. The longitudinal bar, which may be called the basipterygium, is moreover continuous in front with the pectoral or pelvic girdle as the case may be.
Fig. 344. Pectoral fin of a young embryo of Scyllium in longitudinal and horizontal section.
The skeleton of the fin was still in the condition of embryonic cartilage.
b.p. basipterygium (eventual metapterygium); fr. fin rays; p.g. pectoral girdle in transverse section; f. foramen in pectoral girdle; pc. wall of peritoneal cavity.
The primitive skeleton of the pectoral fin is shewn in longitudinal section in [fig. 344], and that of the pelvic fin at a slightly later stage in [fig. 345].
A transverse section shewing the basipterygium (mpt) of the pectoral fin, and the plate passing from it into the fin, is shewn in [fig. 346].
Before proceeding to describe the later history of the two fins it may be well to point out that their embryonic structure completely supports the view which has been arrived at from the consideration of the soft parts of the fin.
My observations shew that the embryonic skeleton of the paired fin consists of a series of parallel rays similar to those of the unpaired fins. These rays support the soft part of the fin which has the form of a longitudinal ridge, and are continuous at their base with a longitudinal bar, which may very probably be due to secondary development. As pointed out by Mivart, a longitudinal bar is also occasionally formed to support the cartilaginous rays of unpaired fins. The longitudinal bar of the paired fins is believed by both Thacker and Mivart to be due to the coalescence of the bases of primitively independent rays, of which they believe the fin to have been originally composed. This view is probable enough in itself, but there is no trace in the embryo of the bar in question being formed by the coalescence of rays, though the fact of its being perfectly continuous with the bases of the rays is somewhat in favour of this view[215].