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Day One
The sperm fertilizes the egg and the cortical reaction occurs preventing another egg from fertilizing. -
Day Two
The cell begins to rapidly divide, but not grow larger. After rapid cleavage, the morula forms. -
Day Three
The blastocyst begins to form with a hollow inside called the blastocoele. A layer of cells called trophoblasts protect the blastocoeles. Inside the blastocyst is an inner cell mass that is exposed to the blastocoele and will later form the embryo. -
Days Four- Five
The blastocyst has reached the uterus and begins to adapt to the new environment. Once it is fully adapted, it can begin to attach itself. -
Days Seven- Twelve
Implantation begins as the blastocyst attaches to the uterine lining. During this time trophoblast cells divide rapidly to create several layers. -
Days Thirteen to Twenty-One
Gastrulation begins which moves cells from the outside surface of the cell to the centerline, this is known as the primitive streak. At this point, the cell now has three layers; the ectoderm, the mesoderm, and the endoderm and a newly created embryonic disk. Lastly, the extraembryonic membranes are forming to support fetal development. -
Days Twenty-two- Fifty-six
From here on out the fetus grows and major organ systems develop.