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The phenomenon of identical and fraternal twins has been a fascinating subject for the biologists from Dalton in 1883 to Newman in 1928 and through it some of the problems of sex and heredity have been clarified. The incidence of mental disease in twins has been widely investigated and during the past decades several studies of psychoses, especially schizophrenia, in identical twins have been made.
Identical twins, also called monozygotic, monochorionic, uniovular, homologous, similar and duplicate, arise from a fission of one fertilized ovum and develop with one placenta and a single chorion. While it has always been presumed that the division of a single fertilized zygote or ovum produced the monozygotic twins, the work of Newman and Patterson in 1910 and 1923 on the armadillo, whose uterus is quite similar to that of man and the primates, substantiated this hypothesis. Arey in 1922 described a pair of twins attached to a single yolk sac which confirmed their uniovular origin.
Monozygotic twins are of the same sex, both males or both females. They are strikingly similar in physical appearance and may be exact counterparts even to the identifying lines of their fingerprints. Because of the mistakes in identity arising from their similitude the mishaps and adventures of identical twins made a droll subject for the playwright as in Plautus' “The Two Menaechmuses” and in Shakespeare's “The Comedy of Errors.”
Fraternal twins, also called dizygotic, dichorionic, binovular, heterologous, dissimilar and pigeon, arise from two fertilized ova. They may be a male and a female or both males and both females. Physically they may be entirely dissimilar or may bear the general resemblance of siblings in the same family. Fraternal twins in their conception conform to the laws of reproduction that govern the plural births of mammals, such as the dog or cat.
The identical twins in the following case report are two boys, college graduates, twenty-three years of age, who came for analysi