A Reference handbook of the medical sciences embracing the entire range of scientific and practical medicine and allied science . n, its significance consumed as food and the ventral swelling is graduallytaken up until no trace of it remains. In the mesodermof the yolk sac blood-vessels develop very early and e.\tend to and from the body of the embrj-o. In some vivip-arous sharks the vascular yolk sac is thrown into foldswhich, by fitting into corresponding uterine folds, alsovascular, constitute a sort of placenta (Balfour). In birds the yolk material is enormously developed,and, though not a
A Reference handbook of the medical sciences embracing the entire range of scientific and practical medicine and allied science . n, its significance consumed as food and the ventral swelling is graduallytaken up until no trace of it remains. In the mesodermof the yolk sac blood-vessels develop very early and e.\tend to and from the body of the embrj-o. In some vivip-arous sharks the vascular yolk sac is thrown into foldswhich, by fitting into corresponding uterine folds, alsovascular, constitute a sort of placenta (Balfour). In birds the yolk material is enormously developed,and, though not at first divided into cells, is still to beregarded as intracellular and as belonging chiefly to theentoderm. The germ layers spread gradually over itssurface (Pig. 504(5, B). The small eml]r_vo sinks into theyolk mass, and the ectodermal layer, with the accom-panying mesoderm, closes in over its back to form theamnion, Arii, and to become cut off from the chorion,Cho. The e.\tra embryonic ccelom consequently becomesdilated. These relations are further illustrated in thecross section. Fig. 5047. From the posterior end of the. Fig. -Diagrams IllustratinK the Development of the Tolk Sac: A, In flsh; B, in birds; C, in the rabbit: D. in man. The embryos are• drawn in blacl;, the ectoderm In heavy lines, the entoderm in light lines, and the mesoderm in dotted lines. The yolk is shaded, ,4 m.,Amnion; Cho., chorion; Coe., extra-embryonic coelom; Y. S., yolk sac. (After Hertwig, Duval, and Minot.) can be appreciated only after examining the correspond-ing structure in other animals. Co5iP.\R.\TivE Embrvologt.—In fish embryos the ento-derm lining the digestive tract is distended ventrally. andits cells are loaded with nutritive material in the form ofgranules. This solid mass of yolk-laden entoderm iscovered on the outside with a layer of mesoderm, and theentire structure is known as the yolk sac. In sharks thesac joins the body by a constricted portion, the jolk stalk(Fig. 5
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