. A manual of zoology. 3S6 MANUAL OF ZOOLOGY SECT. ening, which is moulded, step by step, into the form of the embryo fish. The head, trunk, and tail acquire dis- tinctness, and become more and more clearly separated off from the bulk of the egg, the latter taking the form of a yolk-sac attached by a narrow stalk to the ventral surface of the embryo (Fig. 230). In this condition the various parts of the adult fish can be recognised, but the proportions are different, and the. Fig. 230.—A, embryo of Scyllium with yolk-sac (x \\)\ ]i, under-side of head enlarged. by. f, branchial filament
. A manual of zoology. 3S6 MANUAL OF ZOOLOGY SECT. ening, which is moulded, step by step, into the form of the embryo fish. The head, trunk, and tail acquire dis- tinctness, and become more and more clearly separated off from the bulk of the egg, the latter taking the form of a yolk-sac attached by a narrow stalk to the ventral surface of the embryo (Fig. 230). In this condition the various parts of the adult fish can be recognised, but the proportions are different, and the. Fig. 230.—A, embryo of Scyllium with yolk-sac (x \\)\ ]i, under-side of head enlarged. by. f, branchial filaments protruding through gill-clefts: br. f, branchial filaments projecting through spiracle; , caudal fin; d. ft dorsal fins; e, eye; ex. br. ap. external branchial apertures; mth, mouth; 71a, nostrils; pet. f, pectoral fin: pv. f, pelvic fin; St. yolk-stalk; , ventral fin; yk. s, yolk-sac. (After Balfour, slightly altered.) head presents several peculiarities. The gill-filaments () are so long as to project through the external branchial apertures and the spiracle (fir. /) in the form of long threads abundantly supplied with blood-vessels, and apparently serving for the absorption of nutriment — the albumen in the egg-shell in the case of Scyllium, secretions of the ovi- duct in the viviparous forms. Besides this mode of nutrition the yolk-sac communicates with the intestine by a narrow duct, through which absorption of its contents is constantly. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Parker, T. Jeffery (Thomas Jeffery), 1850-1897; Haswell, William A. (William Aitcheson), 1854-1925. New York, The Macmillan Company; London, Macmillan & Co. ,Ltd.
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