An early stage of mammalian embryo development (typically 5-6 days after fertilization) consisting of a hollow ball of cells with an inner mass and outer layer.
From Greek 'blastos' (bud, germ) + 'kystis' (bladder, sac). Named for its hollow, sac-like appearance under early microscopy.
The blastocyst is arguably the most important stage in your existence—this is what implants into the uterus, and the exact moment it does determines whether pregnancy continues or ends!
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