It stems from the extraembryonic somatic mesoderm on the outer side and the extraembryonic ectoderm on the inner side.

During the third week of development, the lateral mesoderm splits into a dorsal somatic mesoderm (somatopleure) and a ventral splanchnic mesoderm (splanchnopleure).

The chorion consists of two layers: an outer formed by the trophoblast, and an inner formed by the somatic mesoderm; the amnion is in contact with the latter.

It contains the dorsal aorta, genital ridges and mesonephros and lies between the notochord and the somatic mesoderm, extending from the umbilicus to the anterior limb bud of the embryo.

Netrin-1 is found in the floor plate and neuroepithelial cells of the ventral region of the spinal cord, as well as other locations in the nervous system including the somatic mesoderm, pancreas and cardiac muscle.