LIMBS. 



503 



Fig. 345. Pelvic fin of a vert young 



FEMALE ElrfBKYO OF ScYLLIUM STELLAEE. 



bh. basipterygium ; pii. pubic procesB 

 of pelvic girdle ; il. iliac process of pelvic 

 girdle. 



pletely supports the view which has been arrived at from the con- 

 sideration 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 un- 

 paired 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 coa- 

 lescence 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\ 



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 tlie 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 di^awn from it, since 

 I find that the rays of the unpaired fin are similarly segmented from a 

 continuous lamina. In all oases the segmentation of the I'ays 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. 



^ Thacker more especially founds his view on the adult form of the pelvic fins in 

 the cartilaginous Ganoids; Polyodon, in which the part which constitutes the basal 

 plate in other forms is divided into separate segments, being mainly relied on. It is 

 possible that the segmentation of this plate, as maintained by Gegenbaur and Davidoff, 

 is secondary, but Thacker's view that the segmentation is a primitive character seems 

 to me, in the absence of definite evidence to the reverse, the more natural one. 



