_Osteogenesis imperfecta and osteomalacia_ are disorders of dietetic or metabolic origin characterized by fragility of the bones. The latter has developed new significance and importance due to its increased occurrence in almost epidemic form in the Central Empires at the close of the World War. The etiology of both disorders is most obscure. In the future in considering their pathogenesis it will be well to draw a sharp distinction between them. Osteogenesis imperfecta is a disease in which an injury occurs to the foetus without any manifest illness in the mother, which seems to indicate a primary lesion of the foetus (perhaps metabolic); osteomalacia, on the other hand, attacks the mother who gives birth to an apparently healthy foetus. The latter gains added interest because it is a disease of the bones which is attributed to dysfunction of the endocrine glands, to a disturbance especially of the parathyroid. If this observation is confirmed, it furnishes another anatomical basis for associating the glands of internal secretion with pathologic conditions of the bones. One cannot survey the various diseases of the bones--scurvy, rickets, osteoporosis, osteogenesis imperfecta, osteomalacia, etc.,--without realizing that this group is at present in a state of great confusion and will have to be rearranged and winnowed. The differentiation between some of these diseases is not in every instance clear morphologically--for instance, early cases of scurvy and osteoporosis--so that it is difficult to consider them satisfactorily from the standpoint of pathogenesis. Clinical differentiation is still more difficult and uncertain. It seems probable that new entities will be established, as has happened in the past. It is not so long since rickets included almost all the bone diseases of infants. Gradually congenital syphilis, scurvy, and achondroplasia were segregated as distinct diseases. Probably a similar separation of other pathologic conditions, now so entangled as to be indistinguishable, will be evolved.