Is the thecal sac ectoderm?
The dura mater is formed by the mesoderm. The mesoderm is the middle layer of the early trilaminar embryo. The three germ layers are the ectoderm, mesoderm and endoderm, formed by gastrulation. The middle germ layer forms connective tissues and muscle throughout the body, with the exception of the head region, where some of these structures have a neural crest (ectoderm) origin. Connective tissues include cartilage, bone, blood, blood vessel endothelium and dermis, but also the dura mater. The muscles include cardiac, skeletal and smooth muscle (fig. 1).


Fig. 1. Germ layers. Source: UNSW

Source
- UNSW