The development of the epidural space in human embryos.

The epidural space is seen in embryos at stage 17 (41 days) on the periphery of the primary meninx. During stage 18 (44 days) the dura mater proper appears and the epidural space is located between this meninx and the perichondrium and contains blood vessels. During the last week of the embryonic period (stages 20-23) the epidural space is evident around the circumference of the spinal cord. On the posterior surface it is found between the dura mater and the mesoderm of the dorsal body wall.