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ECTODERM

In the cranial one-fourth of the embryo, the ectodermal layer forms a gentle arch from side to side. Caudal to this area there develops a shallow, broad, median furrow called the neural groove, which lies over the notochordal process described below. The floor of the furrow is the neural plate, which is bound on each side by neural folds. The neural plate is the first appearance of the nervous system.

Lateral body folds develop, resulting in the ectoderm clothing the lateral as well as dorsal surfaces of the embryo.

The primitive knot becomes a prominent elevation of ectoderm in the midline at about the middle of the disc as a result of ectodermal proliferation.

The notochordal process extends beneath the ectoderm from the knot to near the cranial end of the disc. The ventral part of the process joins the underlying endoderm throughout most of its extent. The notochordal or neurenteric canal in the process communicates dorsally with the amniotic cavity through an opening caudal to the knot called the primitive pit. Ventrally the canal communicates with the yolk sac cavity through a gap in the endoderm.

The primitive groove becomes a distinct longitudinal furrow extending caudally from the knot in the midline ectoderm.

The two-layered cloacal membrane is caudal to the groove in the angle where the embryo joins the connecting stalk.

ENDODERM

The surface of the endodermal layer contains gentle elevations and depressions having cellular connections with the mesodermal layer.

In the cranial half of the embryo, the midline endoderm joins with the ventral side of the notochordal process.

A thickened area of midline endoderm known as the prochordal plate develops immediately in front of the notochordal process. After formation of the head fold, the plate area will become the cranial end of the foregut. It will fuse with the overlying ectoderm and together will form the oropharyngeal membrane.

Caudal to the cloacal membrane, the endoderm lines a blind, tubular duct called the allantois.

MESODERM

The mesodermal layer spreads laterally from the primitive streak and cranially on each side of the notochordal process. It is present at the cranial end of the disc but is very scant between the prochordal plate and overlying ectoderm.

It can be divided into paraxial and lateral portions.

Numerous junctions are present between the mesodermal and endodermal cells.

At the edge of the disc, embryonic mesoderm is continuous with extraembryonic mesoblasts.

Small, isolated spaces are forming in the cranial mesoderm at the edge of the disc. These spaces represent the first appearance of the pericardial cavity.

Source: Atlas of Human Embryos.