LIMBS. 



613 



FJG. 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. 



be due to secondary development. As pointed out by Mivart, a 

 longitudinal bar is also occa- 

 sionally 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 inde- 

 pendent 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 1 . 



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 



1 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. 



