The development of the spinal accessory nerve in human embryos during 5th week (stages 14 and 15).

The spinal part of the accessory nerve was investigated in serially sectioned human embryos at developmental stages 14 and 15. It has been recognized that the spinal accessory nucleus extends through the upper 4 or 6 cervical segments of the spinal cord. The nucleus is formed by group of cells lying dorsolaterally to the primordium of the ventral horn. There is no continuation of the cells forming the spinal nucleus of the XIth nerve with primordium of the nucleus ambiguus. Extramedullary roots of the spinal accessory nucleus form a long trunk ascending into skull and uniting with vagus nerve. In one embryo at stage 15 the spinal accessory nerve separates at the level of the lower ganglion of the vagus.