A Reference handbook of the medical sciences embracing the entire range of scientific and practical medicine and allied science . then said toliecome villous and perform placental functions. () Except in some rodents and in the marsu-pials, the mesoderm extends completely around the yolksac. In the marsupials the sac is large, almost envelop-ing the embryo and its amnion. It unites with the chor-ion and becomes the fretal portion of the placenta.(Osborn.) The allantois in these animals does not reachthe chorion. In the carnivora both yolk sac anil allau-tois are connected with the c
A Reference handbook of the medical sciences embracing the entire range of scientific and practical medicine and allied science . then said toliecome villous and perform placental functions. () Except in some rodents and in the marsu-pials, the mesoderm extends completely around the yolksac. In the marsupials the sac is large, almost envelop-ing the embryo and its amnion. It unites with the chor-ion and becomes the fretal portion of the placenta.(Osborn.) The allantois in these animals does not reachthe chorion. In the carnivora both yolk sac anil allau-tois are connected with the chorion until birth, and bothare vascular. The horse shows similar relations, but thearea of chorionic connection with the yolk sac becomesvery small, and at birth is merely a scar. (Bonnet.) Inruminants and swine the contact of sac and chorion isslight and transient. In Tarsius and the primates theyolk sac is very small, and in early stages quite distantfrom the chorion (Fig. 5046, B). The extra embryoniccttlom is correspondingly greatly developed. The human 334 REFERENCE HANDBOOK OF THE MEDICAL SCIENCES. Yolk SacVolk Fig. .5(M8.—Ihe Placental Insertionot an Umbllicul Cord, sbuwintSchiiltzes Fold. A part ot theamnion covering the fold has beenreflected, disclosing the yolk sacand persistent vitelline natural size. (AlterLonnberg.) yoik sac is therefore a small and rudimentary organ,neither containing yolk material nor performing placen-tal functions. Gross Anatomy op the Human Yolk Sac.—Theyoungest human embrjosknown, and those long,are attached to a rounded yolk sac haviug a diameterBqual to the length of the embryo and being connected\Yith nearly the whole extent of the intesti-nal tract. The embryo grows faster thanthe yolk sac. At mm. the relations arethose shown in Fig. 4837 in the presentvolume. The opening of the globular yolkSiic now occupies the middle third of theeutodermal tract. A little later the sac be-comes pyriform (Fig. 4
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