The author gives the results of his control measurements of sections from various cortical areas. He used a special block-holder, equipped with a scale, and rotatable, so that from one block of cortex seven or eight planes of section could be made, cutting the cortex at as many different angles. All the measurements were made at the apex of convolutions, where the axes of the majority of nerve-cells pass vertically into the white matter. The depth increments referable to obliquity are tabulated as percentages of the shortest measurement. He concludes that, using sections cut and mounted with extreme care to avoid undue obliquity, one may expect an error of something under 6 per cent., of which almost one half is due to difficulty in determining the line of demarcation between cortex and white matter. The fixing of this line is somewhat arbitrary, owing to the rather gradual and straggling manner in which the spindle-cells of the lowest cortical layer disappear as the white matter is reached.