||This animation shows folding of the embryonic disc beginning week 3 of development.
Embryonic disc (midline section) shown to the left and early placenta to the right. The embryonic disc dorsal (ectoderm) top and ventral (endoderm) bottom. Cranial end of disc to the left and caudal end of disc to the right.
Note also the early cardiac region shown at the cranial end of disc and the allantois at the caudal end of disc extending into the connecting stalk.
Folding of the embryonic disc
- brings the cardiac region and allantois ventrally
- brings the buccopharyngeal membrane, cloacal membrane and connecting stalk ventrally
- encloses the embryo in amniotic cavity
- narrows the connection between yolk sac and midgut of the gastrointestinal tract to a channel, the yolk stalk or vitelline duct
- blue - epiblast layer of amniotic cavity
- yellow - endoderm layer of the yolk sac and gastrointestinal tract
- red - outside embryo extraembryonic mesoderm and inside embryo mesoderm
Compare this sectional view of folding with the Amniotic Cavity view, filling the amnion and yolk cavities.