The extraembryonic mesoblast 5 is formed already during the 2nd week. It is involved in the construction of the placenta and the other appendage organs.
The intraembryonic mesoblast is the third germinal layer. It arises in the 3rd week via the immigration of cells at the primitive streak. Out of this develop the various tissues and organs of the embryo.
In the beginning, the cells of the mesoblast (mesodermal cells) build a thin, widely meshed layer on both sides of the median line, between the ectoderm and the endoderm.
While the notochord is forming - it grows to the same extent that the primitive streak recedes - the intraembryonic mesoderm cells multiply on both sides of the median line and so form 3 structures in the shape of longitudinal columns.
This process begins at the cranial pole and continues up to the 4th week in the caudal direction. In the following section we shall see which specific structures arise from these three formations.