Human Embryo Carnegie stage 8 (Week 3, 17 - 19 days, 1.0 - 1.5 mm)
Features: neural plate, brain fold, neural groove, amniotic sac, presomitic mesoderm, embryonic disc, primitive node, primative streak, primative groove, connecting stalk
View: embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side. Amniotic membrane removed, brain fold top and connecting stalk bottom.
Events: Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm.
Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes. These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract.
From the primitive node a tube extends under the ectoderm in the opposite direction to the primitive streak. This tube forms first the axial process then notochordal process, then finally the notochord.
The notochord is a key to embryonic folding and regulation of ectoderm and mesoderm differentiation. It lies in the rostrocordal axis and the embryonic disc will fold either side ventrally, pinching off a portion of the yolk sac to form the lining of the gastrointestinal tract.