Bilaminar blastocyst or Bilaminar disc refers to the epiblast and the hypoblast, evolved from the embryoblast. These two layers are sandwiched between two balloons: the primitive yolk sac and the amniotic cavity.
The inner cell mass, the embryoblast, begins to transform into two distinct epithelial layers just before implantation occurs. The epiblast is the outer layer that consists of columnar cells.The inner layer is called the hypoblast, or primitive endoderm, which is composed of cuboidal cells. As the two layers become evident, a basement membrane presents itself between the layers. The final two layers of the embryoblast are known collectively as the bilaminar embryonic disc as well as the bilaminar blastocyst or bilaminar blastoderm. This bilaminar blastocyst also defines the primitive dorsal ventral axis. Blastocyst implantation will occur during the second week of fetal development in the endometrium of the uterus; the epiblast is dorsal and the hypoblast is ventral.
Read more about Bilaminar Blastocyst: Formation of The Blastocyst, Becoming Bilaminar, Establishment of The Amniotic Cavity, Formation of The Yolk Sac and Chorionic Cavity, Epiblast Cells During Gastrulation