The developing embryo implants into the uterine lining at about 5 days after fertilization. At this stage, it is a fluid-filled ball of cells called a:

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At 5 days after fertilization, the embryo is in the blastula stage, which is characterized by a fluid-filled ball of cells called a blastocyst. The morula stage occurs before the blastula stage and is a solid ball of cells, while the gastrula stage occurs after the blastula stage and is characterized by the formation of three germ layers. The epiblast and hypoblast are both formed during the blastula stage and are important for the development of the embryonic disk. However, neither of these terms refers to the blastula itself.

The Developing Embryo Implants into the Uterine Lining at About