The pathogenesis of congenital vertebral malformations was investigated histologically in 266 human embryos and fetuses. Malformations were found in 11 specimens, and were grouped according to a detailed classification. All the malformations were observed as abnormalities of the cartilaginous anlage of the vertebral body, and permitted some conclusions as to their pathogenesis. A hemivertebra or other defect of formation of the vertebral body is considered to be caused by abnormal differential growth of the loose-celled area. Defects of segmentation are due to complete chondrification of the dense-celled area or to an absence of the dense-celled area. The notochord does not seem to be responsible for malformations. Compensatory growth of other vertebral bodies resulting from a partial defect or a peculiar shape of the body is already present in this very early stage of development. Some specimens show a relationship between the abnormal distribution of the intersegmental arteries and the abnormality of the vertebral segments. Considering the importance of the intersegmental artery in the formation of the definitive vertebral body anlage, as mentioned in a previous paper, it may be concluded that congenital vertebral malformations are likely to occur during the stage of resegmentation and to be related to the abnormal distribution of the intersegmental arteries.