. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 104 COMPARATIVE ANATOMY median fins, and these can still be recognised in young embryos of Elasmobranchs (Fig. 81, A) and to a less extent in those of Sturgeons, Teleosts, and Amphibians. They extended backwards along the sides of the body from just behind the head, gradually converging towards the anal region, where they became continuous with the ventral part of the median fin-fold (Fig. 81, A), and thus resemble the lateral or metapleural folds present in the adult Amphioxus. As is usually the case in the median fin
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 104 COMPARATIVE ANATOMY median fins, and these can still be recognised in young embryos of Elasmobranchs (Fig. 81, A) and to a less extent in those of Sturgeons, Teleosts, and Amphibians. They extended backwards along the sides of the body from just behind the head, gradually converging towards the anal region, where they became continuous with the ventral part of the median fin-fold (Fig. 81, A), and thus resemble the lateral or metapleural folds present in the adult Amphioxus. As is usually the case in the median fins (p. 102), certain parts of these lateral folds have undergone reduction, only the anterior and posterior portions remaining to form two paired (pectoral and pelvic) fins or limbs, which must therefore be Fig. 81, A.—Transverse Section through the Embryo of .i Shark (Pri4iurm melanof!tomns), 9 mm. L(ing, showing the Mode of Origin of the Pec- toral Limb-Buds [ap.). ch, notochord ; co, ccelome ; myomeres, seen to be growing ventrally; my, spinal cord. looked upon as the localised remains of a continuous lateral fin-fold on either side of the body, and as being homodynamous (, serially homologous) structures. Into these paired fins the myotomes extend, and cartilaginous supports (pterygiophores) are formed from the mesoblast, as in the case of the median fins. These radii appear first of all at the base of the fin, gradually extending centrifugally into the latter, and also, becoming fused, centripetally into the body-wall. An articula- tion is then secondarily formed between the fused basal part of the skeleton situated in the free portion of the limb (hasiptei^gium) and that which extends into the lateral body-wall and serves as a support for the limb proper : this latter portion constitutes the litnb-arcli or girdle. The arch may remain comparatively small and not extend. Please note that these images are extracted from scanned page images that may have been digita
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