http://scienceweek.com/2000/sw000121.htm
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*morula/blastocyst stage: The "morula" is an early
embryonic *cleavage stage consisting of a solid mass of cells
(blastomeres). The next stage is the "blastula" stage, in which
the cells form a hollow sphere. The "blastocyst" is an
early form
of the blastula stage, a mammalian egg in the later stages of
cleavage. The blastocyst consists of a hollow fluid-filled ball
of cells and an inner cell mass (embryonic stem cells) from which
the embryo develops.
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http://en.wikipedia.org/wiki/Blastula
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In mammals, blastulation leads to the formation of the
blastocyst, which
must not be confused with the blastula; even though they are similar in structure, their cells have different fates. The blastocyst consists of two primary cell lines: the inner cell mass, also known as the epiblast, and the trophoblast. The former is the source of embryonic stem cells and gives rise to all later structures of the adult organism. The latter combines with the maternal endometrium to form the placenta in eutherian mammals.
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