The separation from the hypoblast of two lateral masses of mesoblast has already been described.
These have supposed that the mesoblast is derived from the epiblast.
The first of these forms the hypoblast, and the latter the mesoblast.
The outer of these is the epiblast, the inner the mesoblast.
The mesoblast cells in the region of the body are formed in the same way as before, viz.
From the mesoblast are formed all the remaining parts of the body.
Between the epiblast and hypoblast there is a layer of mesoblast, which does not extend as far as the two other layers.
They are coated by epiblast and contain a core of mesoblast.
From this region the mesoblast spreads as a continuous sheet to the sides and posterior part of the blastoderm.
Two of the most obscure points in the development are the origin of the mesoblast in the trunk, and of the body cavity.
mesoblast mes·o·blast (měz'ə-blāst', měs'-)
The middle germinal layer of an early embryo, consisting of undifferentiated cells destined to become the mesoderm.