. Outlines of zoology. Zoology. 494 FISHES. wards share in the making of the embryo. On the other hand, it has been said that they do nothing, even that they are the heads of numerous spermatozoa which have suc- ceeded in entering the ovum. At the close of segmentation the blastoderm is a lens- shaped disc with two strata of cells. It is thicker at one end—where the embryo begins to be formed. Towards the other end, between the blastoderm and the yolk, lies a seg- mentation cavity (Fig. 164, ). At the embryonic end, the outer layer or epiblast under- goes a slight invagination (Fig. 164, x
. Outlines of zoology. Zoology. 494 FISHES. wards share in the making of the embryo. On the other hand, it has been said that they do nothing, even that they are the heads of numerous spermatozoa which have suc- ceeded in entering the ovum. At the close of segmentation the blastoderm is a lens- shaped disc with two strata of cells. It is thicker at one end—where the embryo begins to be formed. Towards the other end, between the blastoderm and the yolk, lies a seg- mentation cavity (Fig. 164, ). At the embryonic end, the outer layer or epiblast under- goes a slight invagination (Fig. 164, x.), beginning to form the roof of the future gut {g.), in other words establishing the hypoblast. This inflected arc of the blastoderm corres- ponds to the blastopore or mouth of the gastrula, which is. Fig. 164.—Elasmobranch development. (After Balfour.) Uppermost figure shows blastoderm at an early sta^e. Ef., epiblast ; s,s:.c., segmentation cavity ; «., yolk nuclei. Middle figure shows the invagination which forms the gut. .t'., blastopore; g., archenteron. Mesoderm dark. Lowest figure, a longitudinal section at a later stage. Ep., epiblast ; , neural canal ; , neurenteric canal ; .n" .'ut ; n notochord. Mesoderm dark. much disguised by the presence of a large quantity of yolk. As the invagination proceeds, the segmentation cavity is obliterated. The floor of the gut is formed by infolding of the lateral walls. Along the mid dorsal line of the epiblast a medullary groove appears —the beginning of the central nervous system. Its sides afterwards arch towards one another, and meet to form a medullary canal (Fig. 164,). A posterior. 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 Thomson, J. Arthur (John Arthur), 1861-1933; Newbigin, Marion I. (Marion Isabel), 1869-1934; Smith, Preser
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
