THE GASTROINTESTINAL TRACT possesses functions in addition to digestion; this concept, based on morphological evidence, stems, like the discovery of the first hormone secretin (8), from the beginning of the century. Masson (92) in 1914 attributed an endocrine function to the previously described chromafin (69), osmiophilic, basigranular yellow, or acidophil cells of the gastrointestinal mucosa.Reinforcement for this endocrine concept of gastrointestinal function was given as early as 1938 when Feyrter (53) described the existence of a group of "specialised, clear cells" diffusely dispersed throughout the body. Feyrter's clear cells were stained lightly by conventional histological methods (Fig. 1) and reacted to silver salts (showing argyrophilic and argentafinity properties). Feyrter (54) FIG. 6. Electron micrograph of somatostatin-containing cell from human gut. Cell is pear-shaped and extends from basement membrane to gut lumen where it terminates with microvilli (M). Large (250-300 nm), spherical, electron-dense secretory granules ( G ) are concentrated in basal half of cell. Glutaraldehyde-osmium fixation, Araldite embedding. Bar, 1 pm.