. Outlines of zoology. Zoology. 670 MAMMALIA. or inner mass, which is at first a solid aggregate of cells (Fig. 290, ), becomes a sac, as a morula .may become a blastosphere. The upper part of this sac forms the lining of the incipient gut, while the lower portion, following the contour of the blastocyst wall, forms the lining of the um- bilical vesicle (cf. the Chick). From this vesicle or yolk-sac the embryo becomes folded off, and the connection between the two is narrowed, just as in the chick, into a canal—the vitelline duct, which is part of the " umbilical cord," entering
. Outlines of zoology. Zoology. 670 MAMMALIA. or inner mass, which is at first a solid aggregate of cells (Fig. 290, ), becomes a sac, as a morula .may become a blastosphere. The upper part of this sac forms the lining of the incipient gut, while the lower portion, following the contour of the blastocyst wall, forms the lining of the um- bilical vesicle (cf. the Chick). From this vesicle or yolk-sac the embryo becomes folded off, and the connection between the two is narrowed, just as in the chick, into a canal—the vitelline duct, which is part of the " umbilical cord," entering the embryo at the future navel. (4) Between the epiblast and the hypoblast of the embryo, the mesoblast develops, split- ting into an outer, parietal, or somatic, and an inner, visceral, or splanchnic layer. The cavity between these is the incipient body cavity. A double fold of somatic mesoblast, carrying with it a single sheet of epi- blast, rises up round about Fig. 294. — Development of foetal membranes.—After Hertwig. Uppermost figure shows up-growth and down-growth of amnion folds. E., Embryo; , amnion fold ; a1., amnion proper ; a2., subzonal membrane ; the gut; y., umbilical vesicle or yolk- sac. The dotted line represents meso- derm ; the dark, hypoblast. The second figure shows origin of allantois, and the amnion folds have met. The third figure shows increase of allantois (eil.)\ the dwindling yolk-sac (); , amniotic cavity; , subzonal membrane. The fourth figure shows the embryo apart from its membranes; ;«., mouth; a., anus. Note umbilical connection with 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. Edinburgh, Glasgow and London, H. Frowde and Hodder & Stoughton
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
