Free blastocyst (stage 3)

From around the 32-cell stage, liquid-filled cavities form which gradually coalesce to create a single cavity called the blastocyst (stage 3). This consists of a coat of cells (trophoblast) covering the blastocyst cavity as well as the inner cell mass, i.e. the embryoblast. The cells of the embryoblast form one pole of the blastocyst, and the cavity the other pole. This sets up a polarized structure. The embryoblast now consists of two layers: the epiblast (near the trophoblast) and the hypoblast (bordering the blastocyst cavity). This two-layered embryoblast is called the embryonic disc. The epiblast forms embryonic cells and the hypoblast forms extra-embryonic structures. The formation and positioning of epiblast and hypoblast defines the dorso-ventral axis.
At the end of stage 3, the zona pellucida dissolves ("hatching" of blastocyst). The trophoblast now becomes the new protective envelope for the embryo, consisting of two portions: the polar trophoblast, which is in direct contact with the embryoblast, and the mural trophoblast, which sheaths the blastocyst cavity.