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Human Embryo - Carnegie Stages 7

Carnegie stages 7 Week 3, 15 - 17 days, 0.4 mm

Features: embryonic disc, primitive node, primative streak, primative groove, yolk sac

View 1: embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side.

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.

Stages 7 Links: Carnegie stages 7 | Gastrulation | Notochord | Week 3

• Embryo Stage 7 (dorsal)
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  Dorsal view

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  Primitive streak and node

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  Oral and cloacal membranes

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  Axial mesoderm

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  Paraxial mesoderm

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  Intermediate mesoderm

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  Lateral plate

Reference

Image Source: UNSW Embryology

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