About the time morula enters the uterine cavity fluid begins to penetrate through the zona pellucida into the intercellular spaces of the inner cell mass .
Gradually the intercellular spaces becomes confluent and finally a single cavity the blastocoel forms .
At this time the embryo is a blastocyst.
The cells of the inner cell mass now called the embryoblast are at one pole and those of the outer cell mass or trophoblast flattens and forms the epithelial wall of the blastocyst .
The zona pellucida has disappeared allowing implantation to begin .
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