The wall of the primary yolk sac consists of extraembryonic endoderm thought to originate from the embryonic endoderm and migrate around the inside of the blastocystic cavity. When the membrane is complete it forms the new cavity. Thus the blastocyst cavity is converted into the primary yolk sac and between the endoderm of the yolk sac and the trophoblast, extraembryonic reticulum and extraembryonic mesoderm may be observed. Later spaces forming within the extraembryonic mesoderm will coalesce to from the chorionic cavity.
The formation of the primary yolk sac cavity is the defining feature of stage 5b
The mechanisms and precise sequence of events in the formation of the primary yolk sac is subject to much discussion. See O'Rahilly R., Müller F., (1987) Chapter 5 and Vögler H., (1987)
The secondary yolk sac consists of an inner layer of extra-embryonic endoderm and an outer layer of extra-embryonic mesoderm which is, or will soon be, continuous with the layer of extra-embryonic mesoderm that lines the trophectoderm shell. These two mesoderm layers become separated by the chorionic cavity.
The precise mechanism of secondary yolk sac formation is subject to debate, the most likely mechanism is an opening up and limited disintegration of the primary yolk sac membrane with the incorporation of some remnants of the membrane into the secondary yolk sac while the remainder of the primary sac can be seen as vesicles in the chorionic cavity. See Vögler H.,(1987) pp 81-88 for discussion
Blood islands in the yolk sac were noted by Hertig A.T. (1935) in several 6a embryos such as Carnegie Embryo No.6734 (Yale) and in many stage 6b embryos. Jones H.O., Brewer J.I. (1941) mention yolk sac blood islands in their description of Carnegie Embryo No. 8819.
A yolk-sac diverticulum is sometimes present and may represent a primitive form of the allantois
The yolk-sac wall is trilaminar in places, the intermediate layer appearing mesenchymous. O'Rahilly R., Müller F. (1987) p 56
The opening of the yolk sac into the gut narrows during this stage so that a yolk sac stalk begins to appear as seen in the 29 somite embryo Carnegie No. 1062 compared to the 14 somite embryo, Carnegie No. 4529. O'Rahilly R., Müller F., (1987) Figs 12.3 &11.7
The opening of the midgut into the yolk sac has become restricted to a long slender stalk sometimes called the vitelline or vitello-intestinal duct. O'Rahilly R., Müller F., (1987) p152
The connection with the embryo via the yolk sac stalk is severed at a variable time during this, the previous or the next stage. The remains of the yolk sac stalk become incorporated into the umbilical cord while the yolk sac itself forms a small remnant between the chorion and amnion at the distal end of the umbilical cord.