In recent years several writers have attempted to bring the known aetiological effect of maternal age in mongolism into direct relationship with its causation. Brousseau (1928) sifted the available data very carefully, but failed to come to any conclusion as to how maternal age could exert its effect. Benda (1947) considers that advanced age is only a subsidiary factor making maternal illness or endocrine disorder more significant, and a similar view seems to be held by Ingalls (1947). The conclusion is drawn by Geyer (1939) that mongolism is due to an abnormal ovum; if so, the older the mother, presumably the more likely are ova to be abnormal. Conversely, Engler (1949) believes that the foetal dysplasia is caused by faulty embedding of a normal ovum in uterine mucosa which has deteriorated in consequence of age, infection or surgical interference. Jenkins (1933) lays stress on the analogy with certain phenomena seen in animal genetics, for Wright (1926) had shown that the manifestation of hereditary Polydactyly in guinea-pigs was influenced by the age of the dam: young mothers produced more polydactylous offspring than older ones. A similar effect has been found by Holt (1947) in polydactylous mice. There is no experimental evidence as to the nature of the mechanism involved, but it may be supposed to be physical or chemical. The search for a process akin to antigenic incompatibility has also been suggested (Penrose, 1946), immunity developing with greater ease in maturer than in younger maternal tissues. It is not necessary to look for a specifically pathological process, however, since the maternal-foetal reaction might be analogous to that in the aetiology of fraternal twins, which especially occurs at late maternal ages, which centre around 38 years.