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 



my. 



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 (i.e., 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 



