BY WILLIAM A. HASWELL, M.A., B.SC. 81 



deduced the theory that in their simplest form the paired fins of 

 fishes are simply continuous lateral folds similar in their derivation 

 to the unpaired fins, the continuous fold of each side becoming 

 subsequently differentiated into the anterior and posterior fins. 



From the formation of the pelvic plexus, some of the nerves 

 going to which in FJasmobranchs are derived from vertebra', 

 segments situated considerably in front of the fin, Davidoff * 

 deduced the conclusion that the limbs had moved backwards from 

 an orginally anterior position, and regards the facts which he 

 adduces as favouring Gegenbaur's hypothesis. If, he supposes, 

 the paired fins were derived from continuous lateral folds, they 

 would be developed in the place they were ultimately to occupy, 

 and there would be no trace of any previous shifting backwards 

 or forwards. Gegenbaur f also has endeavoured to show that 

 the discovery of the lateral ridges supports rather than overthrows 

 his theory, — the ridges being persistent embryonic structures 

 marking the passage backwards of the pelvic fins. 



Thacher (Proc. Connecticut Academy), J and, independently of 

 him, Mivart (Trans. Zool. Soc. IX.) have sought to prove that a 

 comparative study of the structure of the median and paired fins 

 of adult Elasmobranchs and Ganoids leads to the same conclusion 

 as the study of development, viz., that the paired and unpaired 

 fins are strictly homologous structures, and are not developed by 

 the modifiation of any pre-existing portion of the skeleton. 



In a subsecpient memoir § Balfour discusses the general bearings 

 which he regards his embryological observations on Scyllium to 

 possess upon the theory of the nature of limbs, pointing out that 

 these observations are much more favourable to Thacher' s and 

 Mivart's views than to those of Gesrenbaur and Davidoff. 



* Beitrage zur vergleichenden Anatomie der hinteren Gliediuasse der Fische, llorphol 

 Jahrbuch, V., pp. 450-520 (1879.) 



t Zur Gliedniassen-Frage, Morph. Jahrb. V., pp. 521-526 (1879.) 



I I only know this memoir as quoted by Mivart, Balfour, and Davidoff. 



§ On the Development of the Skeleton of the Paired Fins of Elasmobranchii considered in 

 relation to its bearings on the Nature of the Limbs of the Vertebrata, P.Z.S., 1881, p.p. 

 656-670. 



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