endomeninx

noun

en·​do·​me·​ninx ˌen-də-ˈmē-niŋ(k)s  -ˈmen-iŋ(k)s 

plural endomeninges -mə-ˈnin-(ˌ)jēz 

: the layer of embryonic mesoderm from which the arachnoid coat and pia mater of the brain develop