engleski

Human embryo before ultrasound visualization

Gestation begins with the first day of bleeding of the last menstrual period preceding fertilozation. The average gestation is usually divided into several stages. The pregestation stage includes the days of the last menstruation and the days preceding fertilization. The pre-embryonic stage comprises fertilization, repeated divisions (cleavage) of the oocyte and formation of the hollow sphere known as the blastocyst. After implantation, the inner cell mass of the blastocyst differentiates into external and internal germ layers known as ectoderm and endoderm. Formation of the three germ layers, between days 7, and 20, is known as gastrulation and followed by embryonic organogenesis.

Human embryo; embryonic development; human development; implantation; circulation development

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