‘... not all patients showing a considerable number of gametocytes in the peripheral blood will give rise to an infection when bitten by a susceptible insect host (James, 1926)... not only must these sexual forms be sufficiently numerous in the peripheral blood but they must also be in a state of mature development. For these reasons... it is necessary to make a study of the relative prevalence and character of the gametocytes (more especially of the male ones) present in the peripheral blood at the time the insects are fed (James, 1931b). In practice, the suitability is determined by making counts of the number of gametocytes which exflagellate under appropriate experimental conditions, as described by James (1934).’ (12)