Congenital malformations of the flipper in three West Indian manatees, Trichechus manatus, and a proposed mechanism for development of ectrodactyly and cleft hand in mammals.

Three cases of congenital ectrodactyly of the flipper in the manatee are described, including one case of bilaterally-symmetrical cleft hand. A hypothesis assumes that a defect in the apical ectodermal ridge (AER) in the developing hand plate of the early embryo is the initiating factor in the development of ectrodactylous and cleft hand malformations in man and other mammals. Variations in the site, extent, and time of the AER defect will account for many of the morphologic variations observed in these congenital malformations.