OF THE PAIRED FINS OF F.LASMOBRANCHS. 721 



divide the entire skeleton of the fin into three segments pro-, 

 meso-, and metapterygium. 



"The metapterygium represents the stem of the archiptery- 

 gium and the rays on it. The propterygium and the mesop- 

 terygium are evidently derived from the rays which still remain 

 attached to the shoulder-girdle." 



Since the publication of the memoirs of Thacker, Mivart, and 

 myself, a pupil of Gegenbaur's, M. v. Davidoff 1 , has made a 

 series of very valuable observations, in part directed towards' 

 demonstrating the incorrectness of our theoretical views, more 

 especially Thacker's and Mivart's view of the genesis of the 

 skeleton of the limbs. Gegenbaur 2 has also written a short 

 paper in connection with Davidoff's memoir, in support of his 

 own as against our views. 



It would not be possible here to give an adequate account of 

 Davidoff's observations on the skeleton, muscular system, and 

 nerves of the pelvic fins. His main argument against the view 

 that the paired fins are the remains of a continuous lateral fin 

 is based on the fact that a variable but often considerable 

 number of the spinal nerves in front of the pelvic fin are united 

 by a longitudinal commissure with the true plexus of the nerves 

 supplying the fin. From this he 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 Davidoff. 



1 M. v. Davidoff, " Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. 

 Fische, I.," Morphol. Jahrbuch, Vol. V. 1879. 



2 "Zur Gliedmaassenfrage. An die Untersuchungen von Davidoff's angekniipfte 

 Bemerkungen," Morphol. Jahrbuch, Vol. v. 1879. 



