. Outlines of zoology. d escapeby a pair of pores opening together behind the anus. Development.—The ova of the haddock, like those ofother Teleosteans, contain a considerable quantity of yolk,.are fertilised after they have been laid, and undergomeroblastic segmentation. The eggs float, are pelagic;while those of the herring sink, are dimersal. At one pole of a transparent sphere of yolk lies a disc of formativeprotoplasm of a light terra-cotta colour. The ovum is surrounded by afirm vitelline membrane. After fertilisation the formative disc dividesfirst into two, then into four, th
. Outlines of zoology. d escapeby a pair of pores opening together behind the anus. Development.—The ova of the haddock, like those ofother Teleosteans, contain a considerable quantity of yolk,.are fertilised after they have been laid, and undergomeroblastic segmentation. The eggs float, are pelagic;while those of the herring sink, are dimersal. At one pole of a transparent sphere of yolk lies a disc of formativeprotoplasm of a light terra-cotta colour. The ovum is surrounded by afirm vitelline membrane. After fertilisation the formative disc dividesfirst into two, then into four, then into many cells, which form theblastoderm. From the edge of the blastoderm certain yolk-nuclei orperiblast-nuclei are formed, which afterwards have some the end of segmentation the blastoderm lies in the form of a doublyconvex lens in a shallow concavity of the yolk. The blastoderm extends for some distance laterally over the yolk ;the central part raises itself, and thus forms a closed segnienlalion.
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
