Chapter 33. The Fetal Membranes - Review of Medical Embryology Book

THE TROPHOBLAST, during the morula stage, begins to differentiate from the superficial cellular layer of the fertilized egg
BY THE BLASTOCYST STAGE, at about day 5, the trophoblast is differentiated (described with the placenta since it gives rise to it)
IMPLANTATION begins about day 6
THE AMNIOTIC CAVITY, which is hollowed out from the middle of the inner cell mass (embryoblast) makes its appearance with the beginning of the primitive yolk sac at about day 7 and is bounded by entoderm and Heuser's membrane. By day 9, the primitive yolk sac is clearly seen

The external layer is fused to the trophoblast to form the chorion
The internal layer is attached to the amnion and with it forms the somatopleure and with the yolk sac forms the splanchnopleure
Between the 2 layers appears the extraembryonic coelom, except for the region of the connecting stalk where the embryo is connected to the wall of the egg

THE AMNIOTIC CAVITY enlarges and is carried along by the edges of the embryonic disk during cephalocaudal flexion
THE PRIMITIVE YOLK SAC is entirely bordered by entoderm which develops along Heuser's membrane
THE ALLANTOIS appears at the union of the caudal portions of the disk and the primitive yolk sac

THE EXTRAEMBRYONIC COELOM gets smaller, and the amniotic cavity enlarges at the expense of the coelom
THE PRIMITIVE YOLK SAC constricts and the yolk sac appears, attached to the area of the primitive gut by the future vitelline duct
THE ALLANTOIS progresses in the connecting stalk, along with the umbilical-allantoic vessels

THE EXTRAEMBRYONIC COELOM disappears as it is affected by the development of the amniotic cavity
THE YOLK SAC is found up against the placental area, at the end of the long vitelline duct which later regresses
THE ALLANTOIS, after being extended over almost the entire length of the umbilical cord, disappears distally. The umbilical vessels, however, continue to develop

THE YOLK SAC has disappeared almost completely
THE UMBILICAL CORD now contains only the umbilical vessels and remnants of the allantois and vitelline duct
THE AMNIOTIC CAVITY continues to grow until term, at which time it contains almost a liter (1000 ml) of liquid - the so-called "bag of waters"
AT TERM, the membranes surround the placenta with the cord and fetal vessels in its center. The membranes are torn during labor in order to permit delivery