The meninx primitiva refers to the mesenchymal covering of the brain from which the arachnoid mater, pia mater and dura mater are formed in mammals and birds. Both the dura mater (pachymeninges), and arachnoid and pia mater (leptomeninges) are largely mesodermal in origin, although apparently the innermost layer of the pia is neuroectodermal. Normally resorption occurs between the 8th and the 10th week of gestation.
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