Fig. 345. Pelvic fin of a very young female embryo of Scyllium stellare.
bb. basipterygium; pu. pubic process of pelvic girdle; il. iliac process of pelvic girdle.
A point may be noticed here which may perhaps appear to be a difficulty, viz. that to a considerable extent in the pectoral, and to some extent in the pelvic fin the embryonic cartilage from which the fin-rays are developed is at first a continuous lamina, which subsequently segments into rays. I am however inclined to regard this merely as a result of the mode of conversion of the indifferent mesoblast into cartilage; and in any case no conclusion adverse to the above view can be drawn from it, since I find that the rays of the unpaired fin are similarly segmented from a continuous lamina. In all cases the segmentation of the rays is to a large extent completed before the tissue in question is sufficiently differentiated to be called cartilage by an histologist.
Thacker and Mivart both hold that the pectoral and pelvic girdles have been evolved by ventral and dorsal growths of the anterior end of the longitudinal bar supporting the fin-rays.
There is, so far as I see, no theoretical objection to be taken to this view, and the fact of the pectoral and pelvic girdles originating continuously, and long remaining united with the longitudinal bars of their respective fins is in favour of rather than against this view. The same may be said of the fact that the first part of each girdle to be formed is that in the neighbourhood of the longitudinal bar (basipterygium) of the fin, the dorsal and ventral prolongations being subsequent growths.
The later development of the skeleton of the two fins is more conveniently treated separately.
Fig. 346. Transverse section through the pectoral fin of a young embryo of Scyllium stellare.
mpt. basipterygial bar (metapterygium); fr. fin ray; m. muscles; hf. horny fibres.