. Outlines of zoology. Zoology. 498 PISCES—FISHES, are covered with large Graafian follicles, each containing an ovum. The ripe ova burst into the body cavity, and enter the single aperture of the oviducts, which are united an- teriorly just behind the heart. About the middle of each oviduct there is a large oviducal gland, which secretes the " purse " ; the elastic lower portions open into the cloaca. Development.—The ripe ovum which bursts from the ovary is a large sphere, mostly of yolk, with the formative protoplasm concentrated at one pole. The formation of polar bodies (maturat
. Outlines of zoology. Zoology. 498 PISCES—FISHES, are covered with large Graafian follicles, each containing an ovum. The ripe ova burst into the body cavity, and enter the single aperture of the oviducts, which are united an- teriorly just behind the heart. About the middle of each oviduct there is a large oviducal gland, which secretes the " purse " ; the elastic lower portions open into the cloaca. Development.—The ripe ovum which bursts from the ovary is a large sphere, mostly of yolk, with the formative protoplasm concentrated at one pole. The formation of polar bodies (maturation) takes place at an early stage. Fer- tilisation occurs in the upper part of the oviduct. Some observers have described the occurrence of Fig. 213.—Elasmobranch develop- ment.—After Balfour. ;' Uppermost figure shows blastoderm at an early stage. ;Ep,, Epjblas^j', segmentation-cavity ; »,, yolk-nuclei. Middle figure shows the invagination which forms the gut. x., Blastopore ; g-., archenterpn. VM*sbderm .dark. Lowest figure, a longitudinal section at a later stage. £#;, Eplblast; «.c, neural canal; , neurentericcanal; g., gut; «., notochord. Mesoderm dark. As the ovum descends further, it is surrounded first by albuminous material, and then by the four-cor- nered " mermaid's purse " secreted by the walls of the oviducal gland. This purse is composed of keratin— a common skeletal substance which occurs for instance in hair and nails. Its corners are produced into long elastic tendrils, which may twine round sea-weed, and thus moor the egg. Rocked by the waves, the embryo develops, and the young skate leaves the purse at one end. The egg-case of some sharks, the Port Jackson shark (Cestracion Philippi) has elastic spiral fringes, and is found securely wedged among the rocks ; that of a neighbour species (C. galeatus) has reduced spirals ending in a couple of tendrils, which may be 90 in. in length, and serve very effectively t
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
