The subarachnoid space develops early in the human embryonic period

Abstract

The anlage of the subarachnoid space is seen in embryos at stage 14 (33 days) in the innermost zone of the primary meninx as irregular spaces on the ventral surface of the spinal cord. At first this space is only on the ventral surface of the spinal cord.
From stage 18 (44 days) on, when the dura mater proper is formed, the reticular tissue of the primary meninx and spaces are around the circumference of the spinal cord. These spaces gradually coalesce and contain many blood vessels.