Zygote-formation is the initial step in the formation of every embryo, and this zygote is a single cell formed from the union of two gametes (male and female) as a result of fertilization.
Each embryo starts to develop as a zygote. This zygote is a single cell that is formed as a result of the union of the male and the female gametes (the male sperm cell fertilizes the female egg cell). The developing human hence is usually referred to as an embryo up until the ninth week after conception. Post this, it is referred to as the fetus.
Therefore, in the initial stage of the development of the embryo, the single-celled zygote experiences several quick cell divisions known as the cleavage in order to form the blastula. This resembles a cluster of cells. Then, cells in a blastula-phase embryo begin to align themselves into layers, phenomena known as gastrulation. The layers hence produce various parts of the developing multicellular entities such as the tissues, organs and the organ system.
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