Early Development of the Conceptus
Before implantation of the blastocyst is complete, a small, fluid-filled space appears within the cluster of blastomeres forming the inner cell mass (embryoblast); this space is the amniotic cavity. As the amniotic cavity enlarges, the remaining blastomeres of the inner cell mass become rearranged to form a flattened disk consisting of two layers of simple epithelium arranged back-to-back; this structure is the bilaminar embryo. Differentiation of the embryonic disk allows the developing conceptus to be divided into embryonic and extraembryonic areas; subsequent developmental changes will continue to accentuate the boundaries between embryonic and extraembryonic regions of the conceptus.
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