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acrosomal reaction

release of enzymes from the head of the sperm, which causes the sperm to penetrate the zona pellucida of the ovum, allowing fertilization to take place


human life stage spanning the transition between childhood and adulthood. It involves rapid physical and emotional changes


human life stage in which an individual has reached full physical and mental maturity, typically starting at 20 years and ending at death


extraembryonic membrane consisting of endoderm that grows out from the yolk sac. It incorporates into the umbilical cord and urinary bladder


extraembryonic membrane forming a transparent sac that fully encloses the embryo. Formed from the embryonic disc, it contains amniotic fluid

amniotic fluid

clear fluid that surrounds and cushions the embryo within the amnion and enables movement and development


structure consisting of an inner cell mass, the embryoblast, surrounded by the trophoblast, an outer squamous cell layer


one of the two daughter cells produced by the first cleavage of the zygote following fertilization


outermost extraembryonic membrane, formed from the trophoblast. It forms the fetal portion of the placenta and secretes hormones, removes wastes, and provides fetal nutrition

chorionic villi

branching protrusions of the embryo's syncytiotrophoblast that penetrate progressively deeper into the endometrium, providing extensive contact with maternal blood


rapid mitotic divisions of the zygote during the three days postfertilization that produce cells called blastomeres


developing human from week three through the end of the eighth week of gestation


occurs in the final stage of gestational development. It extends from week nine until birth and involves major growth and differentiation of organ systems


the formation of a structure from the blastula that has three distinct layers of cells and body axes


carrying and development of an embryo—which forms into a fetus—in the uterus, from fertilization until childbirth

gonadotropin-releasing hormone

hormone that triggers the secretion by the pituitary of follicle-stimulating hormone (FSH) and luteinizing hormone (LH)


human life stage beginning at the end of the neonatal period—at approximately six weeks—and ending at around two years


spherical bundle of 16 or more cells produced by successive divisions during the preembryonic stage

neonatal period

the first six or seven weeks of life when the individual undergoes drastic changes


temporary vascular organ that normally forms on the uterine wall and through which nutrients and oxygen diffuse from the maternal bloodstream to the developing embryo or fetus


stage of development when the human body becomes capable of reproduction


degeneration of organ systems that takes place between the age of peak physical form and death


outer squamous cell layer of a blastocyst

umbilical cord

tissue cord connecting the embryo or fetus to the placenta through three blood vessels that deliver oxygen and nutrients and remove wastes

yolk sac

extraembryonic membrane that forms a small sac and contributes to digestive tract development and formation of blood, sperm, and egg cells


fertilized egg—the single-celled union of the male gamete, or sperm, with the female gamete or egg