Congenital duplication.

Almost all types of congenital duplication described in humans have been observed in food animals, and a number are illustrated in this article. Congenital duplications, especially conjoined symmetric twins, are relatively common defects in cattle. The incidence is less in sheep and in hogs, and they appear to be rare in goats. In cattle, cranial duplication is predominant; this is in contrast to caudal duplication in sheep and hogs. These differences among species are interesting and may explain why congenital duplications appear more frequently in cattle than in other domestic animals. Based on the number of eyes and ears, classification criteria for cranial duplications are proposed. The cause of congenital duplications is not known.