myelospongium

noun

my·​e·​lo·​spon·​gi·​um ˌmī-ə-lō-ˈspän-jē-əm, -ˈspən- 

plural myelospongia -jē-ə 

dated

: a network in the embryonic central nervous system derived from the spongioblasts and giving rise to the glia