Bilaminar blastocyst

Bilaminar blastocyst or Bilaminar disc refers to the epiblast and the hypoblast, evolved from the embryoblast. ^{[1] [2]} These two layers are sandwiched between two balloons: the primitive yolk sac and the amniotic cavity.

The cells of the inner cell mass which face the blastocyst cavity form a single layer of primitive cells called the hypoblast. The cells at the margin of the hypoblast will begin to multiply and spread with the margins of the hypoblast growing together to create a sac, called the primitive yolk sac (later during gastrulation, the hypoblast is replaced by endoderm. Consequently, the hypoblast makes no direct tissue contribution to the fetus.)

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