A rare case of parasitic head protruding from the right side of the face of a male patient is presented. The 35-year-old patient had a parasitic head with an abnormal brain, two eyebrows, two underdeveloped eyes, a nose, a mouth, 12 teeth, a tongue and plenty of hairs. The parasitic head was excised. The histological study of serial sections of the accessory brain reveals that the latter is composed of a mass of nervous tissue consisting of neuroglia and nerve cells. These cells are evenly distributed in some areas and scattered as isled in others. There is no demarcation of the different parts of the brain and various layers are ill-defined. The arrangement of various type of cells is not orderly. The histological elements could be seen as follows: underdeveloped meninges and leptomeninges, well-developed typical somato-motor cells with Neissl bodies within the cytoplasm, ill differentiated nerve cells, typical and atypical neurons, and atypical primary sensory neuron, typical nerve ganglion, nerve roots, neural tube, brain sands etc.

According to the clinical features and serial sections of the accessory brain, it seems possible that the formation of parasitic head occured in the early stage of the embryo. That is to say the premordia established just after the formation of neural plate or primitive streak and the encephalo-vesicle established before the neural plate. During this period a piece of internal cell mass was separated from the cephalic region of embryo at the level of otic vesicle, rotated 90° and developed its own encephalo-vesicle and branchial arches. The accessory brain received nutrition from the main head. The presence of synchronous movement of the two heads may be explained by that the blood and-nerve supply of the accessory head originated chiefly from the main head. The accessory head movement must have its own nervous control, but it was disguised by the movement of the main head; if it had no its own nervous control, those nerve cells seen in the accessory brain would have degenerated.