82 STUDIES ON THE ELASMOBRANCH SKELETON, 



" If Gegenbaur's view were correct we should expect to find in 

 the embryo, if anywhere, traces of the second set of lateral rays ; 

 but the fact is that, as may easily be seen by an inspection ot 

 figures 6 and 7, such a second set of lateral rays could not possibly 

 have existed in a type of fin like that found in the embryo. With 

 this view of Gegenbaur's it appears to me that the theory held by 

 this anatomist to the effect that the limbs are modified gill-arches 

 also falls, in that his method of deriving the limbs from gill-arches 

 ceases to be admissible, while it is not easy to see how a limb 

 formed on the type of the embryonic limb of Elasmobranchs could 

 be derived from a gill-arch wiih its branchial rays."* 



He also points out that Huxley's view that the proximal piece 

 of the axial skeleton of the limb of Ceratodus is the mesopterygium, 

 and that the fin of the Elasmobranchs is derivable from that of 

 Ceratodus by the drawing in of the axis, is negatived by the proof 

 afforded by the facts of embryology of the secondary character and 

 late development of the inesopterygium. He shows also that the 

 arrangement of the nerve-plexuses as described by Davidoff does uot 

 necessarily require the explanation given by that anatomist. Tbe 

 fact that some of the nerves which go to form the pelvic plexus 

 are derived from vertebral segments in front of the position of the 

 fins may be explained by a previously greater extent of the fin, 

 just as well as by its movement backwards, f 



Finally Owen (Proc. Zool. Soc , 1883) has recently given a short 

 summary of his views on the subject of the homologies of the 

 vertebrate limb, and has shewn how his theory of the origin of 

 limbs from lateral appendages of haemal arches gaius support from 

 Balfour's investigations on the development of the fins of the 

 Elasmobranchii. 



* I. c. p. 669. 



t I have ventured (On the Structure of the Paired Fins of Ceratodus. Proc. Linn. Soc, 

 N.S.W., Vol. VII., p. 10.) to make the very obvious suggestion that the derivation of the 

 pectoral and pelvic plexuses from a number of spinal nerves was a strong piece of evidence 

 in favour of Balfour's theory and against that of Gregenbaur ; but I am now inclined to 

 think, in view of certain facts observed by Furbringer (Morphologischea JahrbuBch, IX.) as 

 to the origin of the nerves supplying the pelvic fin in some Teleostei with thoracic or 

 jugular pelvis fins, that the position of the spinal nerves from which the plexuses are 

 derived is too plastic a factor to support any wide generalisatioE at all. 



