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.