For fertilization to occur, sperm cells must be released in the vagina during the period that the egg cell is alive. The sperm cells move through the uterus into the fallopian tube, where one sperm cell may fertilize the egg cell. The fertilization brings together 23 chromosomes from the male and 23 chromosomes from the female, resulting in the formation of a fertilized egg cell with 46 chromosomes. The fertilized cell is a zygote.
The zygote undergoes mitosis to form two identical cells that remain attached. This takes place about 36 hours after fertilization. Mitosis then occurs more frequently. Soon a solid ball of cells, a morula, results. Morula formation occurs about six days after fertilization. During that time, the cells are moving through the fallopian tube. Within the next two days, a hollow ball of cells called a blastocyst forms. The blastocyst enters the uterus. At one end of the blastocyst, a group of cells called the inner cell mass continues to develop.
About eight days after fertilization, the blastocyst implants itself in the endometrium of the uterus. During implantation, the outer cells take root in the endometrium. This outer layer of cells, called the trophoblast, gives rise to projections that form vessels. These vessels merge with the maternal blood vessels to form the placenta. The trophoblast also develops into three membranes: the amnion, the chorion, and the yolk sac membrane.
The inner cell mass undergoes changes to form three germ layers known as the ectoderm, the mesoderm, and the endoderm. The ectoderm becomes the skin and nervous system, the mesoderm becomes the muscles and other internal organs, and the endoderm becomes the gastrointestinal tract. The embryo is formed at about the fourth week when all the organs of the body have taken shape.