. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. DEVELOPEMENT OE FISHES. 609 TTcll as the internal 425 This, as a rule, is the form antl structure acquired by the tail in existing Teleostomous Fishes: but the 'heterocercal' modification does not intervene between the proto- and homo-cereal ones in the GadidcR. The pectoral fins are developed usually before extrication, and are often of large relative size : in this respect, as well as in the inferior position of the mouth, in the unsymmetrical form of the tail, in the gristly skeleton, and uncovered gill-slits, the emb
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. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. DEVELOPEMENT OE FISHES. 609 TTcll as the internal 425 This, as a rule, is the form antl structure acquired by the tail in existing Teleostomous Fishes: but the 'heterocercal' modification does not intervene between the proto- and homo-cereal ones in the GadidcR. The pectoral fins are developed usually before extrication, and are often of large relative size : in this respect, as well as in the inferior position of the mouth, in the unsymmetrical form of the tail, in the gristly skeleton, and uncovered gill-slits, the embryo Salmon, Pike, Perch, &c., manifest transitory characters which are permanent in Sharks. The singular productions of the rostrum in most Plagiostomes, like the elongation of the jaws in osseous species, are later phenomena of developement. It is interesting to find the broad, depressed, obtuse embryonic form of head common to many of the Fishes of the old red-sandstone. M. Agassiz thus accounts for the extreme rarity of the Ichthyolites of this formation presenting a profile view of the head: it lies in most cases upon the upper or the under surface of the body. All the Plagiostomes have the external as division of the vitcllicle, fig. 425 ; the peduncle of the external one d, is longer, in some species con- siderably so, than in Osseous Fishes, and it is beset with villi in Carcliarias and Zygana} The tegumcntary covering of the outer yolk, ib. d!, is denser and more ojiake in Plagiostomes: the inner yolk, ib. e, is co- vered only by the proper vitel- line tunic, which is thin and transparent: it communicates with the short tract of small intestine which intervenes be- tween the pylorus and the val- vular straight gut, li: it receives the external yolk, d', as this is ijroo-ressively squeezed into the abdomen by the contraction and interstitial absorj)tion of its tunics, c': and, as no part of the fo3tal abdominal appendage is cast off, nor the chord divided, there is n