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embryo
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Diagram showing early human embryo development. The top row illustrates ovum cleavage from a single cell to a morula. The bottom row shows blastocyst formation and implantation, with labeled cell structures. embryos of different animals Labeled diagram of an amniotic egg showing the embryo surrounded by membranes including the amnion, chorion, yolk sac, and allantois, with albumin, amniotic fluid, yolk, and shell also identified. Diagram showing the development of external genitalia in human embryos, with labeled illustrations comparing undifferentiated, female, and male stages at 12 and 36 weeks, highlighting anatomical changes during fetal growth. Three labeled cross-section diagrams show the development of a human embryo implanted in the uterine lining at 7.5, 13, and 23 days, highlighting structures such as the embryonic disk, amniotic cavity, yolk sac, trophoblast, and chorion. Two labeled diagrams show the human embryonic disk at 18 days: a three-quarter view and a cross-section, highlighting structures such as the amniotic cavity, yolk sac, embryonic shield, body stalk, and layers including ectoderm, mesoderm, and endoderm. Diagram showing three stages in early human embryonic development: the growth of the amnion at 23 days, a back view of an embryo at 21 days with the amnion cut open revealing neural structures, and a side view at 23 days with the yolk sac and amnion cut open, highlighting the heart, brain, spinal cord, and umbilical blood vessels. Diagram showing human embryological and fetal development in three stages: at six weeks, the embryo is enclosed within halved amnion, chorion, and uterus; another view at six weeks highlights the formation of the umbilical cord; at three months, the fetus is shown within the halved amnion, chorion, and uterus, with the placenta and umbilical cord labeled. Cross-sectional medical illustration of a human uterus at seven weeks of pregnancy, showing the developing embryo inside the amniotic sac, placenta, umbilical cord, uterine wall layers, cervix, and vagina, with labeled anatomical structures related to gestation and embryology.
Follow the development of rice-paddy fish from fertilization to hatching in a laboratory setting

embryo: References & Edit History

Article Contributors

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  • The Editors of Encyclopaedia Britannica
    Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.

Other Encyclopedia Britannica Contributors

Article History

Type Description Contributor Date
Add new Web site: Live Science - Should we rethink our legal definition of a human embryo? Sep 12, 2026
Add new Web site: CiteSeerX - The cleavage stage embryo (PDF). Apr 06, 2025
Cross-references added. Jan 24, 2025
Add new Web site: CORE - Toward Guidelines for Research on Human Embryo Models Formed from Stem Cells. Formed from Stem Cells. Nov 30, 2024
Add new Web site: Reproductive BioMedicine Online - Current Issue - Cryopreservation of animal and human embryos by vitrification. Oct 15, 2024
Add new Web site: BBC News - Scientists grow whole model of human embryo, without sperm or egg. Jan 11, 2024
Add new Web site: United States Conference of Catholic Bishops - What is an Embryo? Nov 30, 2023
Add new Web site: National Center for Biotechnology Information - PubMed Central - Re-defining the human embryo: A legal perspective on the creation of embryos in research. Oct 05, 2016
Article revised and updated. Nov 08, 2007
Added new Web site: The Visible Embryo. May 30, 2006
Added new Web site: The Visible Embryo. May 30, 2006
Article added to new online database. Jul 20, 1998
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