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The first through eighth weeks after fertilization are called the embryonic.

Week 1

• Within a day, the zygote begins mitotic cell division (cleavage) forming blastomeres. By the 4th day, the blastomeres have formed a solid ball called a morula.
• The morula enters uterine cavity around day 4 or 5.
• Glycogen-rich secretions from the endometrial glands nourish the developing morula.
• By day 5, the embryo has shed the zona pellucida and become a hollow ball of cells known as the blastocyst.
• Within the blastocyst, new cells move to form the inner cell mass and trophoblast cells.
• By day six, the blastocyst implants into the endometrium.

Week 2
• During the second week after fertilization, the implanted blastocyst stage continues to develop.
• On the eighth day, the trophoblast cells of the blastocyst differentiate into two layers:
• The inner cytotrophoblast layer.
• The outermost syncytiotrophoblast layer secretes enzymes that promote further implantation as well as human chorionic gonadotropin (hCG), a hormone that acts to maintain the endometrium.
• Also on the eighth day, the cells of the inner cell mass differentiate into two layers:
• The hypoblast layer.
• The epiblast layer.
• Both layers together form the bilaminar embryonic disc.
• During the ninth day, the epiblast cells begin to multiply and migrate, forming the fluid-filled amniotic cavity.
• At the same time, the multiplying hypoblast cells begin to develop into a yolk sac.
• As a result, the bilaminar embryonic disc is positioned between the amnion and yolk sac.
• By the twelfth day, differentiation of yolk sac cells forms an extraembryonic mesoderm.
• This new layer, along with the syncytiotrophoblast and cytotrophoblast layers, form the chorion.
• The chorion grows to form the connecting stalk, which is the future umbilicus.
• The chorion, destined to form the embryonic part of the placenta, is important in the exchange of nutrients between maternal blood and the embryo.

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