58 MR. F. M. BALFOUR ON THE SKELETON 



concludes that the pelvic fin has shifted its position, and 

 that it may once therefore have been situated close behind 

 the 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 Davidofi". 



Gegenbaur, in his paper above quoted, further urges 

 against Thacher's 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,^ 

 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 later into twelve parts, the number 

 of which subsequently becomes reduced. 



The observations recorded in the present paper 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 procuring material, my observations 

 are confined to the two British species of the genus Scyllium, 

 viz. Sc. stellar e and Sc. canicula ; yet I venture to believe 

 that the results at which I have arrived are not wholly 

 without interest. 



* G. 'Swirski, * Untersuch. iib. d. Entwick. d. Schultergurtels u. d. 

 Skelets d. Bruslflosse d. Hechts./ Inaug-. Diss,, Dorpat, 1880. 



