| Pre-embryonic period |
First 2 weeks |
• Fertilization |
| • Cleavage and blastocyst formation |
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| • Implantation |
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| • Formation of embryoblast and trophoblast |
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| • Differentiation of embryoblast to two-layered (bilaminar) germ disc |
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| • Differentiation of trophoblast into cytotrophoblast & syncytiotrophoblast |
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| Embryonic period |
3-8 weeks |
• Trilaminar germ disc formation with the formation of 3 germ layers |
| • Early organogenesis |
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| • Formation of extraembryonic supportive organs and membranes such as placenta, umbilical cord, amnion, and allantois |
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| Fetal period |
9 weeks to birth |
• Growth and specialization of body structures |