Pituitary glands of chicken, from stages 20 (70 ∼ 72 h of incubation) to 46 (20 days) of Hamburger and Hamilton (1951), were studied by immunocytochemical and histological stainings and India ink injection into blood vessels. Using the distribution pattern of 6 types of immunoreactive adenohypophyseal cells and the location of pituitary stalk as guideposts, we found how specific areas in the epithelium of Rathke’s pouch differentiate into specific regions of the adenohypophysis at 20 days. In the sagittal plane, the walls of Rathke’s pouch were tentatively divided into the upper part (A₁ + A₂) and lower part (A₃) of the anterior wall, and the posterior wall (P₁ + P₂ + P₃). The cephalic lobe was mainly assembled by the proliferation of parenchymal cells in the areas A₂ + A₃ + P₂ of Rathke’s pouch epithelia at 3 days of incubation. The caudal lobe was derived from A₁ + P₁ + P₃. The pars tuberalis was derived from A₁ + A₂. Thus, the avian adenohypophysis is established at 13 days, though the blood supply to the pars distalis is established at 20 days. Therefore, the cephalic lobe and caudal lobe of the pars distalis and the pars tuberalis of the chicken adenohypophysis are derived from specific areas of the cell cords of Rathke’s pouch at 3 days of incubation.