The development and differentiation of the ventro-caudal part of the diencephalon, the saccus infundibuli, has been examined in salmon. Salmo salar L., and in pike. Esox lucius L. In salmon, the saccus infundibuli becomes the saccus vasculosus but in pike, this does riot occur. The question of saccus vasculosus and Part of the neural lobe of the tetrapod pituitary being homologous is dealt with.