The identity of the germ cell tumours of the pineal and the thymus with those of the testis and ovary suggests that the widely disseminated primordial germ cells might subserve some special function in these sanctuaries. It is proposed that thymic localization might be required for the conveyance of genetic haematological and immunological information and that the pineal–diencephalic localization could programme neuroophthalmic tissues prior to the development of the blood–brain barrier. The latter speculation was tested by producing allergic encephalomyelitis in thymectomized, bursectomized and thymobursectomized chickens. It was found that although thymectomy and, to a lesser extent, bursectomy decreased the severity of the experimental encephalomyelitis the combined procedure resulted in more severe inflammatory lesions. This may be due to release of suppressed intraneural immunological mechanisms in the somatically impaired b