662 MR. F. M. BALFOUR ON THE SKELETON [Juue 7, 



visceral arches. Granting, however, that DavidofF's deduction from 

 the character of the pelvic plexus is correct, there is, so far as I see, 

 no reason in the nature of the lateral-fin theory why the pelvic fins 

 should not have shifted ; and, on the other hand, the longitudinal 

 cord connecting some of the ventral roots in front of the pelvic fin 

 may have another explanation. It may, for instance, be a remnant 

 of the time when the pelvic fin had a more elongated form than at 

 present, and accordingly extended further forwards. 



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 oi Polyodon is a primitive type; and also suggests that the 

 epithelial line which 1 have found connecting the embryonic pelvic 

 and pectoral fins in Torpedo may be a rudiment indicating a migra- 

 tion backwards of the pelvic fin. 



With reference to the development of the pectoral fin in the 

 Teleostei there are some observations of 'Swirski^ which unfortu- 

 nately 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. 



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 pro- 

 curing 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 1 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 attached 

 to the side of the body. The cartilaginous skeleton only occupies 



^ G. 'Swirski, ' Untersuch. iib. d. Eatwick. cl. Schultergiirtels u. d. Skelets 

 d. Brustflosse d. Hechts.' Inaug. Diss. Dorpat, 1880. 



