. Elementary text-book of zoology. Zoology. 128 VERTEBRATA. The development of the embryo (tig. 580) begins with a total or partial (discoidal) segmentation. The first rudiment of the embryo is usually a germinal disc or blastoderm lying upon the yolk. From the posterior end of this disc the alimentary cavity is developed. A primitive streak which marks the long axis of the embryo is developed by a thickening of the layers of the blastoderm. Two laterally placed longitudinal folds give rise to an ectodermal groove—the medullary groove or first rudiment of the central nervous system—beneath whi
. Elementary text-book of zoology. Zoology. 128 VERTEBRATA. The development of the embryo (tig. 580) begins with a total or partial (discoidal) segmentation. The first rudiment of the embryo is usually a germinal disc or blastoderm lying upon the yolk. From the posterior end of this disc the alimentary cavity is developed. A primitive streak which marks the long axis of the embryo is developed by a thickening of the layers of the blastoderm. Two laterally placed longitudinal folds give rise to an ectodermal groove—the medullary groove or first rudiment of the central nervous system—beneath which is placed the notochord which is developed from the endoderm (fig. 581). The medullary groove which is dilated anteriorly is closed by the growing together of its edges, and the tube so formed gives rise to the spinal cord and to the brain. Its lumen is for some time in Mp. Mv FIG. 582.—Transverse section through a chick embryo of the second day (after Kolliker). Ee, ectoderm; N, medullary canal; End, endoderm; Ch, notochord; UW, proto- vertebra ; UNg, Wolffian duct (primitive duct of kidney); Mp, somatic mesoblast; 3fc, splanchnic mesoblast; Lh, body cavity ; Ao, primitive aorta. communication with the alimentary cavity by the neurenteric canal. At the sides of these structures the mesoderm extends in the form of two bands, the median portions of which (protovertebral plates) become segmented in the course of the further development and give rise to the protovertebrse (figs. 576 and 582). The archinephric duct is separated off at the boundary between the protovertebrse and the unsegmented lateral plates of mesoblast, while the generative glands arise nearer the median line from the peritoneum of the lateral plates of mesoblast. While the dorsal part of the embryo is thus being formed the alimentary canal becomes further developed on the ventral side of the blastoderm, and gradually absorbs the yolk, often leaving an external yolk sac. The young animals only undergo a
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
