. The Cambridge natural history. Zoology. 434 FISHES CHAP. but in viviparous forms, whose embryonic development is completed within special uterine dilatations of the oviducts, additional means of nutrition are provided for the young. Such Elasmobranchs as Spinax, Acanthias, Centrina, Scymnus, Trygon, Torpedo, and Myliohatis have long filaments (villi or trophone- mata) developed from the inner surface of the uterus, which secrete a nutritive fluid, and this fluid is either absorbed by the blood-vessels of the embryonic yolk-sac, or it is taken up by the embryo in some more direct manner. In s
. The Cambridge natural history. Zoology. 434 FISHES CHAP. but in viviparous forms, whose embryonic development is completed within special uterine dilatations of the oviducts, additional means of nutrition are provided for the young. Such Elasmobranchs as Spinax, Acanthias, Centrina, Scymnus, Trygon, Torpedo, and Myliohatis have long filaments (villi or trophone- mata) developed from the inner surface of the uterus, which secrete a nutritive fluid, and this fluid is either absorbed by the blood-vessels of the embryonic yolk-sac, or it is taken up by the embryo in some more direct manner. In some of the Trygonidae and Myliobatidae of the Indian Ocean it seems prob- able that the secretion is taken into the alimentary canal of the embryo either through the mouth or through the open. Fig. 246.—Egg of the Spotted Dog-Fish {Scyllium canimla), showing its mode of attacliuieut after extrusion. (Prom Hertwig, after Kopsch.) spiracles.^ One species, Pteroplatect mierura, has its long and highly vascular and glandular trophonemata gathered into two bundles, which are thrust through the huge spiracles into the pharynx of the embryos, of which there may be from one to three, and the nutritive secretion is apparently digested in the alimentary canal of the embryo and absorbed by the foetal blood-vessels (Fig. 247). A few Sharks, like most species of Mustelus, develop a placenta when the food-yolk in the yolk- sac of the embryo is nearly used up. Folds or projections from the highly vascular wall of the yolk-sac interlock vascular folds of the lining membrane of the uterus, and a diffusion of nutrient material takes place from the maternal blood in the uterine blood-vessels to the foetal blood in the 1 Wood-Mason and Alcook, Proc. Roy. Soc. 49, 1891, p. 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 ori
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