Page 4 - SONO 260 THE FIRST TRIMESTER
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                                       ž The combination of the sperm and ovum produces a structure referred to as the zygote.
ž The zygote undergoes rapid cellular division and eventually forms into a cluster of cells called the morula.
ž The morula continues to differentiate and form a structure referred to as the blastocyst.
ž The outer tissue layer of the blastocyst is comprised of syncytiotrophoblastic tissue, also referred to as trophoblastic cells.
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