Answer :

The embryo is a hollow ball of cells during the blastocyst stage of development.

A blastocyst is a ball of cells that develops early in a pregnancy, approximately five to six days after a sperm fertilizes an egg. It implants in your uterine wall and develops into an embryo, then a fetus. The germinal stage is maintained as the zygote divides into several initial cell divisions to form a solid ball of cells known as a morula (days 3-4). The morula then undergoes additional changes to become a hollow ball of cells known as a blastocyst (days 5-7). The initial stages of multicellular organisms' growth begin with a zygote cell, which then divides rapidly to form the initial cell cluster, or 'blastula.'

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