Failure of all or part of an organ to develop during embryonic growth. Many forms of agenesis are lethal, such as absence of the entire brain (anencephaly), but agenesis of one organ of a pair may cause little problem. Agenesis of a kidney, bladder, testicle, ovary, thyroid, and lung are known. Agenesis of the arms or legs is called meromelia (absence of one or both hands or feet), phocomelia (normal hands and feet but no arms or legs), and amelia (complete absence of a limb or limbs). Agenesis may be caused by absence of embryonic tissue or by chemical exposure in the uterus, and it is often associated with other congenital disorders.

This entry comes from Encyclopædia Britannica Concise.
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