In the study of the so-called primary, or endogenous, mental deficiency, when one excludes well defined pathologic types, such as amaurotic idiocy, tuberous sclerosis and mongolism, a large number of cases may still be found that escape any but a psychometric classification. It can hardly be doubted that these cases constitute a heterogeneous group in which future investigation will individuate new nosologic types, differing essentially in etiology, symptomatology and pathologic character.
An apparently new type of mental deficiency has recently been indicated by Fölling,1 with the term "imbecillitas phenylpyruvica." This author reported having found phenylpyruvic acid in the urine of 10 mentally defective patients, 4 of whom were males and 6 females, ranging from 2 to 20 years of age. In 6 instances the metabolic abnormality was present in two members of a sibship, but it was never found to occur in mentally normal relatives. The mental defect was