The separation from the hypoblast of two lateral masses of mesoblast has already been described.
The outer is the Ectoderm and the inner is the Endoderm or hypoblast.
The first of these forms the hypoblast, and the latter the mesoblast.
Ridge of hypoblast, which will become separated off as the notochord.
In a the epiblast and hypoblast are continuous at the edge of the section, owing to the section passing through the embryonic rim.
It is closed in above and at the sides by the hypoblast, and below by the yolk.
Between the epiblast and hypoblast there is a layer of mesoblast, which does not extend as far as the two other layers.
The upper of these is the epiblast, and the lower the hypoblast.
The line ge in B shews the extension of the hypoblast round the inner side of the vesicle.
To this mass of cells the hypoblast is also partially adherent.
hypoblast hy·po·blast (hī'pə-blāst')