722 DEVELOPMENT OF THE SKELETON 



Gegenbaur, in his paper above quoted, further urges against 

 Thacker and Mivart's views the fact that there is no proof that 

 the fin of Polyodon is a primitive type ; and also suggests that 

 the epithelial line which I have found connecting the embryonic 

 pelvic and pectoral fins in Torpedo may be a rudiment indicating 

 a migration backwards of the pelvic fin. 



With reference to the development of the pectoral fin in 

 the Teleostei there are some observations of 'Swirski 1 , which 

 unfortunately 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 develop- 

 ment 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 

 procuring material, my observations are confined to the two 

 British species of the genus Scyllium, viz. Sc. stdlare and Sc. 

 caniada; yet I venture to believe that the results at which I 

 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. i) ; and it is to be especially noted that the narrowest part 



1 G. 'Swirski, Untersuch. iib. d. Entwick. d. Schultergurtels u. d. Skclets d. 

 Brustflossc d. Hechts. Inaug. Diss. Dorpat, 1880. 



