From the second to the fourth week
After the end of the 2nd week to the 4th week the cell mass from which the embryo arises, experiences numerous modifications. Cell proliferation and migration lead to the process of the mesoblast immigration. This leads to the genesis of a trilaminar germ disk whose structure already exhibits the precursors (anlage) of the most important organs
The processes of cellular proliferation and migration constitute the so-called epithelio-mesenchymal transition (gastrulation in lower vertebrates).
At the end of this period various systems are already differentiated. In particular, a large part of the central (neural tube) and the peripheral nervous systems (neural crest) are developed. The anlage of the muscle system, the skeletal axis and the dermis appear at the level of the somites. The somites consist of mesoblastic, metamerized cell collections.