The yolk-sac (Figs. 22 and 23) is situated on the ventral aspect of the embryo; it is lined by entoderm, outside of which is a layer of mesoderm. It is filled with fluid, the vitelline fluid, which possibly may be utilized for the nourishment of the embryo during the earlier stages of its existence. Blood is conveyed to the wall of the sac by the primitive aortæ, and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins to the tubular heart of the embryo. This constitutes the vitelline circulation, and by means of it nutritive material is absorbed from the yolk-sac and conveyed to the embryo. At the end of the fourth week the yolk-sac presents the appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct. The vesicle can be seen in the after-birth as a small, somewhat oval-shaped body whose diameter varies from 1 mm. to 5 mm.; it is situated between the amnion and the chorion and may lie on or at a varying distance from the placenta. As a rule the duct undergoes complete obliteration during the seventh week, but in about three per cent. of cases its proximal part persists as a diverticulum from the small intestine...
Furthermore, more recent studies indicate that the yolk-sac in humans is vital for the survival of the embryo, Early Human Nutrition:
Early human nutrition and chorionic villous vascularization. Babette A.M. Lisman, M.D., Niek Exalto, M.D., Ph.D.
Department of Obstetrics and Gynecology, Spaarne Ziekenhuis Haarlem and Division of Obstetrics and Gynecology, Academisch Medisch Centrum, Amsterdam
ABSTRACT
The yolk sac plays an active and important role in embryonic nutrition and organogenesis, and therefore can not be considered a vestigial organ. Since the last decade the functional significance of the placental circulation during the first trimester is discussed because of an absent maternal intervillous circulation during the first 12 weeks of pregnancy. The development of the embryo takes place in an oxygen poor environment and the yolk sac turned out to be important for the intercession of embryonic nutrition during the developmental period. Furthermore, the human yolk sac is the main source of numerous proteins and its biosynthetic activity plays an important role in haematopoiesis. Experiments on animals have demonstrated that the yolk sac can be damaged by various substances resulting in embryonic malformations. Ultrasonic studies examining the secondary yolk sac size do not appear to be a sensitive predictor of embryonic integrity and pregnancy outcome. Apart from implantation and organogenesis the development of the placenta tales place during the first 12 weeks of pregnancy but will not be functional until thereafter. The development of the chorionic villous vascular system in first trimester pregnancies is characterized by maturation of luminized vessels from primitive hemangioblastic cords and margination to a situation of peripherally located vessels. Normal chorionic villous vascularization is essential for the undisturbed development of pregnancy. Deficient vasculogenesis may play a role in pathological pregnancy. More clinical studies in this field should be performed to investigate chorionic villous vascularization in complicated pregnancy and its consequences.
From your post: In the human, the "yolk sac" appears only after fertilization adn during embriologic development. It remains a fluid filled sac only briefy before rapidly becoming the primary source of fetal blood cells and vascular conduit.
In view of the above abstract, your contentions that the human yolk-sac is only superficially similar to avian yolk-sacs, and appears only "briefly" before morphing into a structure that becomes "the primary source of fetal blood cells and vascular conduit" is incorrect. According to this research, the yolk-sac is vitally important for the survival of the embryo until the placenta is fully functional after the first 12-weeks of pregnancy.