= Solution
\b[The <amnion>, <allantois> and <yolk sac> are distinct extraembryonic structures whose relations change as the embryo folds and the <umbilical cord> forms.] The historical umbilical vesicle is the <yolk sac>; its duct must not be identified with the allantoic channel.
Early in development the embryonic disc lies between the amniotic cavity dorsally and the <yolk sac> ventrally. The <amnion> has an epithelial lining facing its fluid and an outer supporting mesodermal component. The <yolk sac> has an endodermal lining and an external splanchnic mesodermal covering. Outside them is the <chorionic cavity>, interrupted by the <connecting stalk> linking the embryonic region to the <chorion>. The <chorion> consists of trophoblastic tissues and extraembryonic somatic <mesoderm>; it is not simply another name for the <amnion> or an outgrowth of yolk-sac <endoderm>.
The human <allantois> appears as a small endodermal diverticulum from the caudal yolk-sac region and extends into the <connecting stalk>. With incorporation of the caudal region into the embryo it becomes related to the <cloaca> and developing <urinary bladder>. Its intraembryonic remnant is associated with the <urachus>, later represented by the <median umbilical ligament>. Unlike the large allantoic sac of a bird, the human <allantois> is not an expanding external urine reservoir or an independent aerial respiratory organ.<Coelomic Cavity Development by the University of New South Wales>[Extra-embryonic Coelom; Allantois: cloacal diverticulum, connecting stalk and subsequent bladder relation]{ref}
Embryonic folding converts part of the original endodermal roof of the <yolk sac> into the gut tube. The remaining sac lies outside the embryo and communicates with the midgut through the narrowing <vitelline duct>. Vitelline vessels accompany its early vascular apparatus. The sac supports early nutritional and <blood>-forming functions even though it contains no large yolk store in humans. Its connection normally regresses; persistence of a <vitelline duct> is a different anomaly from persistence of the <urachus>. The presence of a duct alone does not make the two structures interchangeable.<Embryology, Yolk Sac (StatPearls)>[Introduction; Development; Mechanism: umbilical vesicle, midgut connection, vitelline circulation and regression]{ref}
As the <amnion> expands, it surrounds the embryo more completely and encloses the roots of the <connecting stalk> and yolk-sac connection at the umbilical region. Those structures are brought together within an amniotic covering to form the early <umbilical cord>. The cord initially includes the stalk with allantoic remnants and developing umbilical vessels, together with the vitelline connection and associated structures. The mature cord chiefly contains two <umbilical arteries> and one <umbilical vein> embedded in <Wharton's jelly>, beneath an amniotic surface. The vessels arise within vascular mesodermal tissues; it would be inaccurate to say that the endodermal allantoic tube simply turns into all three <blood vessels>.
The <amnion> subsequently comes into apposition and fuses with the <chorion>, greatly reducing the intervening <chorionic cavity>. Its surface continues along the cord towards the fetal placental surface. The <yolk sac> remains outside the amniotic cavity rather than becoming a normal free object floating in amniotic fluid. The mature <amnion> is avascular, whereas the placental <chorion> and the cord have crucial fetal vascular connections.<Embryology, Umbilical Cord (StatPearls)>[Introduction and Development: connecting stalk, vitelline connection, amniotic enclosure and mature vessels]{ref}
These relations can be understood by comparing developmental stages in section: first a disc between two cavities, then a folded embryo with narrowed ventral connections, and finally an expanded amniotic covering around the cord. A section through a later cord cannot by itself reveal every early channel that has regressed. The same membrane names are used across vertebrates, but their relative sizes and functions are not identical in a human fetus and a yolk-rich bird egg.
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