The spinal accessory nerve in human embryos at 6th week (stages 16 and 17).

Investigations were made on serial sections of human embryos at developmental stages 16 and 17 (37-41 days). The spinal nucleus of the accessory nerve presents well delimited cellular group in the dorsplateral part of the ventral horn of the spinal cord. The migration zone is still present. In embryos of stage 16 the spinal accessory nerve joints the fibers originating from the nucleus ambiguous. This takes place at the level of the inferior ganglion of the vagus nerve. In stage 17 the secondary rami of the accessory nerve develop. This is correlated with the differentiation of the sternocleidomastoid and trapezius muscles.