During an experiment on developing rats, we encountered an abnormal rat fetus whose adenohypophysis was not in contact with the brain. This fetus was found and killed on the 21st day of gestation when all types of adenohypophysial hormone-producing cells are already immunohistochemically detectable in the normal gland. When compared with its normal counterpart, this brain-detached adenohypophysis was characterized by: 1) its remarkably small volume, less than 10 percent of the size of the

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... gland; 2) the lack of a pars intermedia; 3) far fewer numbers of corticotropes, somatotropes and thyrotropes; and 4) the absence of prolactin cells. These results are consistent with our previous in vitro data which showed that contact between the adenohypophysial primordium and diencephalon is indispensable for the proliferation and differentiation of the adenohypophysial hormone-producing cells. The absence of the pars intermedia indicates that the formation of this part of the hypophysis is dependent on the brain in rats, as already shown for frogs.