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An embryo is the initial stage of development for a multicellular organism.In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell.
At the beginning of the ninth week, the embryo is termed a fetus (spelled "foetus" in British English). In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. Human embryology is the study of this development during the first eight weeks after fertilization.
1 - morula, 2 - blastula 1 - blastula, 2 - gastrula with blastopore; orange - ectoderm, red - endoderm. Embryology (from Greek ἔμβρυον, embryon, "the unborn, embryo"; and -λογία, -logia) is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and fetuses.
The embryo becomes embedded in the endometrium in a process called implantation. In most successful pregnancies, the embryo implants 8 to 10 days after ovulation. [7] The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception".
The zygote then undergoes a defined development process that is known as human embryogenesis, and this starts the typical 38-week gestation period [a] for the embryo (and eventually foetus) that is followed by childbirth.
The medical definition of an embryo is an organism from fertilization to the beginning of the third month of pregnancy, while a fetus is after that until birth.
The embryo differs from the cells of the mother, and would be rejected as a parasite by the immune system of the mother if it did not secrete, immunosuppressive agents. Such agents include platelet-activating factor , human chorionic gonadotropin , early pregnancy factor , prostaglandin E2 , interleukin-1 alpha , interleukin 6 , interferon ...
Anti-abortion activists argue that embryos are people. For many, the reality is much more complicated.