. Text-book of embryology. Embryology. vertebral arches. They arise, 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 cor
. Text-book of embryology. Embryology. vertebral arches. They arise, 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 Crossopterygi
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Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
