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64-Cell Stage (4 days)

Medically reviewed by: Stephanie Curreli, MD, PhD
Last Updated: Feb 4, 2015

At the 64 cell stage, the embryo reaches the morula stage. At this point, the center of the morula begins to fill with fluid from the uterine cavity. As the space within develops, two distinct types of cells will form. The outer wall will develop trophoblasts and the inner cells will develop embryoblasts.