. The biology of birds. Birds. BIRDS' EGGS 265 begins to be " folded off " from the yolk. As a result of this the embryo is attached by a relatively narrow stalk to the large yolk-sac, over which the extra-embryonic blasto- derm spreads, showing blood-vessels before the end of the first day. The yolk is gradually utilised and, just before hatching, the greatly reduced sac is withdrawn into the body- cavity of the embryo and the navel or umbilical opening closes up. Of great importance are the two embryonic membranes— the amnion which forms a protective hood over the embryo,. Fig. 44.
. The biology of birds. Birds. BIRDS' EGGS 265 begins to be " folded off " from the yolk. As a result of this the embryo is attached by a relatively narrow stalk to the large yolk-sac, over which the extra-embryonic blasto- derm spreads, showing blood-vessels before the end of the first day. The yolk is gradually utilised and, just before hatching, the greatly reduced sac is withdrawn into the body- cavity of the embryo and the navel or umbilical opening closes up. Of great importance are the two embryonic membranes— the amnion which forms a protective hood over the embryo,. Fig. 44.—Diagram of developing embryo of bird (after Grosser). Showing the embryo (e.) ; the inner or true amnion, a protective hood (am.) ; the amniotic cavity containing fluid () ; the extra-embryonic body-cavity () ; the allantois (al.) ; the external serosa includes the outer amnion and the vitelline capsule ; the yolk-sac () full of yolk. and the allantois which spreads as a delicate hood underneath the shell. Blood-vessels from the embryo are distributed abundantly on the allantois, and serve to capture the oxygen which diffuses through the shell from the outer world. Carbon dioxide diffuses out. If the shell be varnished, so that its pores are closed, the embryo must die. The allantois is thus the embryo's breathing organ ; but as it also absorbs some white of egg and receives nitrogenous waste-products from the embryo, it may also be reckoned as a nutritive. 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. New York : Macmillan
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectbirds, bookyear1923
