INTRODUCTION The hypothalamo—hypophysial system has been subjected to a series of intensive structural investigations based primarily on the observations that it contains bundles of non-myelinated fibres, passing from the hypothalamus and terminating in the neurohypophysis (Ramon y Cajal, 1894), and that the fibres originate in specific hypothalamic nuclei (Pines, 1925; Greving, 1926; Meyer, 1935). The identification of pharmacological and physiological activities in extracts of the neurohypophysis (Oliver & Schäfer, 1895; Dale, 1906, 1909; Ott & Scott, 1910; Farini, 1913; von den Velden, 1913) encouraged the morphologists to try to associate their light-microscopic observations with localization of these activities. Herring (1908) had observed basophil bodies in the neurohypophysis (which have come to bear his name) and certain workers proposed that these basophil bodies represent the active principles (Cushing & Goetsch, 1910) which were shown to be hormones by the classical experiments of Verney & Starling(1922); Starling & Verney (1925) and Verney