Animal embryonic development

Animal embryonic development is the series of processes by which a fertilized egg becomes an embryo with organized tissues and body structures.

Cleavage (embryology): Rapid early cell divisions that partition an egg into smaller cells without increasing the embryo’s overall size. Cleavage multiplies embryonic cells before substantial growth begins.
Fertilization: The union of sperm and egg that forms a zygote and combines their genetic material. It begins the developmental sequence by producing the first embryonic cell.
Sea urchin embryogenesis: The development of a sea urchin from fertilized egg through larval stages. Its transparent, externally developing embryo made early cell-fate experiments possible.
Amniotic egg: An egg with extraembryonic membranes that support development on land in reptiles, birds, and monotreme mammals. Its membranes show how some embryos develop outside a mother’s body.
Gastrulation: The process that rearranges embryonic cells into germ layers and establishes the body plan. Gastrulation converts a ball or sheet of cells into an embryo with distinct tissue layers.
Zygote: The single cell formed when an egg and sperm unite. The zygote is the starting cell from which the embryo’s cells descend.
Drosophila embryogenesis: The development of the fruit fly embryo, including formation of its segmented body plan. Fruit-fly embryos exposed genes that establish segment identity and body axes.
Placenta: An organ that mediates exchange between a developing fetus and the mother in many mammals. Placental development supports embryos that remain inside the maternal body.
Neurulation: The formation of the neural tube from embryonic ectoderm in vertebrates. This stage builds the early central nervous system from one germ layer.
Germ layer: A primary embryonic tissue layer that gives rise to particular tissues and organs. Germ layers are the broad tissue divisions established during gastrulation.

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Plant embryonic developmentCompared with: Plant embryos develop within seeds and rely on cell walls rather than animal-style cell rearrangements.