The development of the bilaminar germ disk  3 and the establishment of a feto-maternal blood circulation system were given in detail in the placenta and implantation modules.
In the bilaminar primordium of the embryo (hypoblast or primary endoderm and epiblast) one recognizes in the epithelium of the epiblast a fluid-filled space, the first primordium of the amniotic cavity 5.
Ventrally, the roof of the still incompletely uncovered primary umbilical vesicle 5 (previously the blastocyst cavity) is formed by the hypoblast.
Schematically, amniotic cavity and primary umbilical vesicle together form two hemispheres with two layers (epi- and hypoblast) lying close to one another, thus representing the first embryonic primordium.
However, only the epiblast is responsible for forming the embryo. The hypoblast develops into a part of the extraembryonic appendages. |
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Fig. 1 - Bilaminar embryonic disk,
12 days; stage 5 |
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Legend |
