SUMMARY
The cytological appearance of the pars intermedia of the rabbit both at the light and the electron microscopical (E.M.) levels, is described. Two types of cell,one stainable byperiodic acid—Schiff's reagent and aldehyde—fuchsin and the other chromophilic, non-granular and intercalated between the former, were observed. In E.M. sections the first type had two kinds of granule in its cytoplasm, a few electron-dense about 250 nm in diameter, mostly confined to the Golgi area, and numerous large 'vesicles' or granules of varying density about 500–600 nm in diameter generally distributed throughout the cytoplasm. The second type of cell when viewed in the electron microscope is characterized by microvilli projecting into intercellular spaces, and long, linked processes ramifying in between the principal cells. Varicose nerve fibres and abundant neurosecretory material were demonstrated in the pars intermedia with chrome alum—haematoxylin and Alcian blue. In E.M. sections these neurosecretory granules were similar to those in the infundibular process. At the point where nerve fibres appeared to be making contact with glandular cells, synaptic vesicles of about 50 nm in diameter were also observed.