In 1872 Meynert estimated the number of nerve-cells of the cortex of the convex surface of the cerebral hemispheres in round numbers at 612,000,000. In 1895 Donaldson, taking Meynert's calculation as a starting-point, estimated the total of cortical cells at 1,200,000,000 and in the whole nervous system at 3,000,000,000. In 1895 also, Hammarberg published a very minute and careful study of the size and arrangement of the nerve-cells in the human cortex, which has been already noticed in the Journal. He did not, however, apparently carry out his calculations to the estimation of the total number of cells in the cortex of the brain, and it remained for a woman, Miss Helen Bradford Thompson, to take up the subject and apply Hammarberg's methods to solving the question. Her paper appeared in the June issue of the Journal of Comparative Neurology, and by adopting the data as to dimensions and arrangement