Human Body

Reproduction and Human Development

When do significant changes occur during the early stages of prenatal development?

Several significant changes occur during the first two weeks of prenatal development, as shown in the accompanying table.

Significant Changes in the First Two Weeks of Prenatal Development
Time Period Developmental Stage
12–24 hours following ovulation Fertilized ovum
30 hours to third day Cleavage
Third to fourth day Morula (solid ball of cells is formed)
Fifth day through second week Blastocyst
End of second week Gastrula (germ layers form)

When does the heart begin to beat in a developing embryo?

The heart begins to beat by the twenty-fifth day of development.


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