40 LATERAL FOLD FLNS 



supporting substance the fusion of the cartilages of the 

 hindmost gill bars ; in its outward growth the median axis of 

 the fin was first produced, the minor supports then arrang- 

 ing themselves on both anterior and posterior margins. 

 The fin of Fig. 52 was believed to represent a specially 

 evolved (or "monoserial") form of the archipterygium : the 

 hindmost of its elements, B, was homologized with the 

 primitive fin stem, along whose posterior (post-axial) mar- 

 gin the elements, R, no longer occurred. The structures 

 of Fig. 53 were adduced as a transitional stage in the dif- 

 ferentiation of the biserial archipterygium (Fig. 54) into the 

 monoserial form of Fig. 52. 



The theory of Gegenbaur as to the origin and evolution 

 of the paired fins cannot be said to be in any way generally 

 supported at the present time. The opposing view, that 

 of their derivation from a continuous lateral dermal fin 

 fold, based on the work of Thacher, Balfour, Mivart, 

 Dohrn, Wiedersheim, and others, is widely accepted, and 

 continues to gain supporting evidence on the sides both 

 of embryology and palaeontology. 



In the following discussion of the paired fins the 

 writer has mainly followed the recent studies of Wieders- 

 heim.* 



The paired fins are believed to have arisen as balancing 

 organs, accessory in function to the vertical fins. They 

 probably occurred early in the line of descent as a response 

 to a need for balancing the fish's body, at the time when 

 the vertical fin was separated into caudal, dorsal, and anal 

 elements. There can be little doubt that they first arose 

 in the line of the fish's motion, and are known primitively 

 (Figs. 49, 50), as a pair of keel-like lateral lappets arising 

 somewhat ventrally, and directed outward and downward. 



* Das Glied7nassenskelet der Wirbelthiere^ 1893. 



