. Biology; the story of living things. brain ^pirzccL CarcC digestive, •trcxct/. the yolk sac. Gradually blood vessels develop in the mesenchyme of the yolk sac, facilitat- ing the transportation of food to the develop- ing embryo. Amnion and Chorion In addition to the protection afforded by egg membranes or shells and the yolk sac, the higher vertebrates, namely, the reptiles, birds, and mammals, elaborate additional embryonic membranes that serve not only as supplementary protec- tive devices to keep the embryos from mechan- ical injury but also tem- porarily handle the problems of respirati
. Biology; the story of living things. brain ^pirzccL CarcC digestive, •trcxct/. the yolk sac. Gradually blood vessels develop in the mesenchyme of the yolk sac, facilitat- ing the transportation of food to the develop- ing embryo. Amnion and Chorion In addition to the protection afforded by egg membranes or shells and the yolk sac, the higher vertebrates, namely, the reptiles, birds, and mammals, elaborate additional embryonic membranes that serve not only as supplementary protec- tive devices to keep the embryos from mechan- ical injury but also tem- porarily handle the problems of respiration, excretion, and nutrition. In order to understand their functions, and the fact that their evolution is intimately tied up with that of the land-inhabiting reptiles, birds, and mammals, one must trace their embryological development. As long as organisms returned to the water during the breeding season, as the amphibians still do, the exchange of gases and elimination of wastes takes place directly, since the surrounding water not only contains sufficient dissolved oxygen but also it soon dissipates waste products which are passed through the egg membranes and elimi- nated. With the acquisition of a land habitat, the inability to return to the water to spawn presented new problems, centering about the control of metabolism in the embryo. These needs were met through the elaboration of a series of embryonic membranes, which were apparently developed to facilitate the carrying on of normal metabolic processes through a permeable egg shell. They occur in modified forms in all land vertebrates. SomotopW 5plar2diT7op'l<aure Diagram of a developing fish embryo. Note the " contained " yolk sac. What is its ultimate fate ?. 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 Hunter, George W. (George W
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Keywords: ., bookauthorwalterherberteugeneb1867, bookcentury1900, bookpublish
