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human embryogenesis
Human embryogenesis is the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, human development entails growth from a one celled zygote to an adult human being. Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences. Embryogenesis covers the first eight weeks of development and at the beginning of the ninth week the embryo is termed a fetus.
Human embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is nine months or 38 weeks.
The germinal stage, refers to the time from fertilization, through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days.
During this stage, the zygote, which is defined as an embryo because it contains a full complement of genetic material, begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus. Embryogenesis continues with the next stage of gastrulation when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow. The embryo is referred to as a fetus in the later stages of prenatal development, usually taken to be at the beginning of the ninth week. In comparison to the embryo, the fetus has more recognizable external features, and a more complete set of developing organs. The entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation. A nearly identical process occurs in other species, especially among chordates.
== Germinal stage ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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