In the etiology of mongolism, two problems must be distinct: (1) the causes leading to the abnormal development of the embryo and (2) the abnormal factors operating in the embryo to produce the characteristic growth disorder. The present study deals only with the former problem, the prenatal maternal condition. Since mongolism is present at birth and the characteristic anomalies date back to early stages of fetal development, the causative factors are either of a genetic nature or environmental factors operating in the mother during gestation. In 1939, Benda, Dayton and Prouty1presented a study of 250 pregnancies each resulting in the birth of a mongoloid child. An analysis of the material produced evidence that mongolism is not due to genetic factors. The material indicated also that neither a germ mutation nor a primary inferiority of the ova was an acceptable explanation. These conclusions were based on the following facts: