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What happens during the pre-embryonic stage of human development?
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prenatal development
An ultrasound image of a human fetus.
The pre-embryonic stage represents the first two weeks of human development, characterized by rapid cell division and initial differentiation. This period begins at conception with fertilization, where a male and female gamete unite to form a single-celled [zygote].12
Key processes during this stage include:
- Cleavage: The zygote undergoes mitosis, subdividing into smaller cells called blastomeres. This process occurs as the cluster travels through the uterine tube toward the [uterus].3
- Formation of the Morula and Blastocyst: By the fourth day, the cluster becomes a morula. As it enters the uterus, it develops into a blastocyst, consisting of an inner cell mass—which will become the embryo—and an outer layer called the trophoblast, which contributes to the placenta and fetal membranes.3
- Implantation: The blastocyst eventually attaches to and embeds itself into the endometrium (uterine lining), a process typically completed by the 11th day after fertilization.4
This stage sets the foundation for the subsequent embryonic period, which begins in the third week.1