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