A Reference handbook of the medical sciences embracing the entire range of scientific and practical medicine and allied science . 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. Co5i
A Reference handbook of the medical sciences embracing the entire range of scientific and practical medicine and allied science . 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. 5046, A). The ventral body wall, composed of. Fig. 5047.—Diagrammatic Cross-Section of an Amniote Embryo. Thebody of the embryo is shaded; the extra-embryonic mesoderm isrtprfsented by a dotted line. Sojh, somatopieure; Spl, splanch-uoplcnre: Cfw. chorion; Am, amnion; Coe, ccelom ; In, intes-tine ; iolh, yolk sac. (Minot.) ectoderm with a lining of mesoderm, extends over theyolk sac. but is separated from it by an extension of thebody cavity or ccelom. As the fish grows, the yolk is intestinal tract an out-pocketing projects into the ccelom,in which it lies free. Thisentodermal pocket is the allan-tois, an organ composed of the same layers as the yolksac, and like it, highly vascular. Its competition withthe 3olk sac in placental mammals will be describedpresently. The yolk sac of birds, in later stages, isdrawn into the abdominal cavity, and there it remains inAvater fowl, as a permanent diverticulum at about themiddle of the small intestine. (Oppel.) The mammals present several types
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