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The embroyonic disc is symmetrical and slightly convex in the longitudinal axis in its cranial half.

In the caudal half of the disc the ectodermal and endodermal layers are in contact in the midline. The area of contact is called the cloacal membrane.

Cranial to the cloacal membrane the ectodermal cells are loosely arranged to form the primitive knot.

Extending cranially from the knot, between ectoderm and endoderm, is a median clump of cells known as the noto-chordal or head process.

The primitive streak extends caudally from the knot as a faint zone of ectodermal proliferation in the midline. A shallow furrow, called the primitive groove, forms in the ectoderm over the caudal end of the streak.

A loose sheet of embryonic mesoderm develops lateral and cranial to the streak between ectoderm and endoderm. It probably has three sources: a) ectoderm in the area of the primitive streak, b) endoderm where mesoderm is in close contact and c) extraembryonic mesoblasts.

Source: Atlas of Human Embryos.