Introduction

After the end of the third week, the three germinal layers begin to differentiate and transform so that the initially flat embryonic disk develops into a cylindrical structure like a "C".

The folding and genesis of the abdominal wall permits a delimitation of the embryo, that can now be clearly distinguished from the appending organs. Up to this stage 9 , the extraembryonic tissue went over into the intraembryonic tissue with no boundaries. The folding and the resulting formation of the abdominal wall lead to an enclosure of the mesoderm and the endoderm. They become surrounded by the ectoderm, which later forms the skin.

Two mechanisms lead simultaneously to the formation of the abdominal wall:

• The cephalo-caudal flexion (in the longitudinal direction)
• The lateral folding (in the transversal direction, rolling up)

The cephalo-caudal folding (in the longitudinal direction)

In order to understand how this turning takes place, the structures must first be described that are found in the cephalic end before the folding:
In the cephalic region, rostral to the prechordal plate and the pharyngeal membrane, the mesenchymal cells form the cardiac plate (pericardium) and the septum transversum (which later becomes a part of the diaphragm and separates the coelom into thoracic and abdominal cavities).

With the 180° degree turn that results from the folding, the following occurs:

• the pharyngeal membrane extends towards the lower front (mouth area)
• the cardiogenic plate (which initially lay most cranially) into the thorax area
• between the cardiac anlage and the umbilical vesicle a mesenchymal bridge forms, the septum transversum

After this movement is completed, the brain (encephalon) lies the most cranially, followed by the mouth, heart, and diaphragm (septum transversum).

During this folding the endoderm below the pharyngeal membrane becomes surrounded ventrally by the cardiac anlage. From this region, the throat (pharynx) arises and, subsequently, the thyroid gland, the lungs, and the esophagus. The pharyngeal membrane that for now separates the mouth (ectoderm) from the throat (endoderm) later atrophies.

The folding of the caudal end occurs after the cephalic folding and has the result that the body stalk comes closer to the umbilical vesicle (yolk sac).
Due to the large axial growth the caudal end of the embryonic disk (with the cloacal membrane) comes to lie under the original embryonic disk and thus shoves the allantois and the body stalk in the ventral direction, up to the umbilical vesicle (yolk sac) and merges with its stalk.
To be noted is that now the end of the primitive streak, which initially lies dorsally after the flexion of the embryo, also comes to lie ventrally.