The frequency of monoovular twinning can be roughly estimated by means of the Weinberg method. If such twins could be diagnosed during life, it would be of great value in differentiating the products of inheritance from the influences of environment. The diagnosis, however, meets with considerable difficulties; the author has shown that examination of palm and sole prints is inadequate. It is impossible at present to disregard the evidence from the fetal membranes, but unfortunately correct examination, as to the presence of 1 or 2 chorions, can rarely be obtained. The most recent and perhaps the most promising of the diagnostic methods is that of Siemen, assuming a great number of similarities in physical and mental characteristics. The author has examined 38 pairs of twins whose ages ranged from new-born to 8 years. It was impossible to divide these twins into 2 groups which would have satisfied Siemen''s requirements. Hence the author concludes that Siemen''s method is unsatisfactory for children.