in human physiology, failure of all or part of an organ to develop during embryonic (embryo) growth. Many forms of agenesis are consistently lethal, as when the entire brain is absent (anencephaly), but agenesis of one of a paired organ may create little disruption of normal function. Agenesis of the kidney, bladder, testicle, ovary, thyroid, and lung are known. Agenesis of the long bones of the arms or legs also may occur, called variously meromelia (absence of one or both hands or feet), phocomelia (normal hands and feet but absence of the long bones), and amelia (complete absence of one or more limbs).

Agenesis often occurs because the embryonic tissue that gives rise to the affected organ, or an adjacent embryonic tissue, is missing. At many stages in development, one structure induces the formation of another; removing the first structure causes agenesis of the second. Other cases of agenesis have been associated with chemical exposure in the womb, as in the association between the drug thalidomide and phocomelia.

Agenesis syndromes are frequently associated with other congenital anomalies. In renal agenesis, or Potter's syndrome (absence of one or both kidneys), the ureters also are usually absent, and sex organs may be abnormal. Affected children have wide-set eyes, large, low-set ears, and flattened nose. Agenesis of the lung may be unilateral, a relatively common defect, or bilateral, the latter occurring most frequently with acardia (failure of the heart to develop).