Human BodyReproduction 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.