MEDIAN FIN SKELETON 



347 



vertebral arches. They arise, e.g. in Elasmobranchs, in ontogeny 

 as independent rods of cartilage without definite relation to the 

 metamerism of the body and later on become segmented into 

 three pieces. In those cases, so far as they have been investigated, 

 in which the radial elements are connected with a continuous basal 

 plate of cartilage, this latter appears to arise in ontogeny as a 

 continuous plate, though there is no reason to doubt that it arose in 

 phylogeny by the fusion together of the basal portions of originally 

 separate rays. 



This want of correspondence of the mesial elements of the dorsal 

 fin skeleton with the vertebrae is probably sufficiently explained as 

 a secondary result of the 

 prolonged working of the 

 general principles which 

 have governed the evolu- 

 tion of the median fin 

 and which find their ex- 

 pression in the tendencies 

 (1) of the continuous fin 

 to become specially devel- 

 oped at particular points 

 and to die away in the 

 intervening spaces, (2) of 

 the resulting separate fins 

 to have their base of 

 attachment to the body 

 shortened and (3) of 

 these fins to be situated 

 on the body at the points 

 where they are mechanic- 

 ally most effective. 



Dermal Supports of 

 Median Fins. — The 

 median fins being pri- 

 marily mere extensions of 

 the body in the vertical plane it would only be reasonable to expect 

 that they would show traces of skeletal elements comparable with 

 the placoid elements or their derivatives characteristic of the rest 

 of the surface. And in fact the dermal skeletal supports of the 

 median fins can, some of them, be clearly recognized as homologous 

 with scales, while in others although this may no longer be recog- 

 nizable their origin is found to be closely associated with the 

 basement membrane as was the case with the dermal teeth. 



It will be convenient to consider first of all the dermal skeletal 

 elements in which the direct relation to scales is most clear. Such 

 are the bony fin-rays of Crossopterygian and Actinopterygian fishes. 

 In an ordinary Teleost (Fig. 167) the fin-rays of this type (lepido- 

 trichia, Goodrich) appear in their earliest stage, as shown by 



Fig. 167. — Two successive stages in the development 

 of the lepidotrichia of Salmo. 



A, Salmon (after Harrison, 1893) ; B, Trout (after Goodrich, 

 1904). . b.m, basement membrane ; ect, ectoderm ; I, lepido- 

 trichial rudiment ; mes, mesenchyme of dermis. 



