By the time the fertilized egg reaches the uterus it is a hollow ball of cells called a: Morula
This collection of cells, known as a morula, resembles a little ball by the time it reaches the uterus.
The morula develops a hollow interior and fills with fluid to produce a blastocyst.
The blastocyst will cling to the endometrium, the uterus' lining, by the end of this week. The term for this is implantation.
The zygote, which comes before the blastocyst, is split into the morula, a solid mass with 16–32 blastomeres.
At the 8-cell stage, a process known as compaction starts, in which the spherical, loosely linked blastomeres take on a flattened polarized cell shape.
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