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Day 1
Fertilization occurs. This is the fusion of haploid gametes to form the diploid zygote. Multiple sperm release acrosomal enzymes that create gaps in the corona radiata. Only a single spermatozoon makes contact with the oocyte membrane though, causing membrane fusion which triggers oocyte activation. -
Day 2
Cleavage begins. This is the series of cell divisions that divides the cytoplasm of a zygote. This is when a zygote becomes a pre-embryo, which ultimately develops into a multicellular blastocyst. Cleavage ends once the blastocyst contacts the uterine wall. Cleavage continues through the time the pre-embryo matures into an blastocyst to be implanted in the uterine wall. -
Day 7
This is when implantation begins. Implantation is when the blastocyst adheres to the uterine lining. At the point of contact, the trophoblast cells divide rapidly, making the trophoblast several layers thick. -
Day 14
This is when a third layer of the trophoblast forms through gastrulation. This is done through cells in specific areas moving toward a central line known as the primitive streak. The migrating cells leave the surface and move between two existing layers, creating three layers, the ectoderm, the mesoderm and the endoderm. Gastrulation produces an three-layered oval sheet which is the embryonic disc. This will form the body of the embryo. -
Day 16
This is when the germ layers begin to participate in the formation of four extraembryonic membranes, the yolk sac, the amnion, the allantois and the chorion. The yolk sac is created because it is an important source of blood cell formation. The amnion and the amniotic cavity contains amniotic fluid which surrounds and protects the developing embryo or fetus. The allantois is the base layer which gives rise to the urinary bladder. Finally, the chorion develops a blood supply for the embryo. -
Day 21
This is the beginning stages of embryogenesis. Embryogenesis is the process by which the body of the embryo and its internal organs start to form. It begins as the folding and differential growth of the embryonic disc which projects into the amniotic cavity. These projections are head fold and tail fold. By the end of 10 weeks, embryogenesis is complete and all of the major organ systems are formed. -
Day 23
The heart begins to beat. Blood flow does not begin until around 30 days. This is one of the earliest differentiating and functioning organs in the embryo. The heart forms initially in the embryonic disc and later gets carried to the correct anatomical cavity.
