The<unk>  is a structure integral to the developing nervous system of vertebrate organisms. Located on the ventral midline of the embryonic neural tube, the<unk>  is a specialized glial structure that spans the anteroposterior axis from the midbrain to the tail regions. It has been shown that the<unk>  is conserved among vertebrates, such as zebrafish and mice, with homologous structures in invertebrates such as the fruit fly Drosophila and the nematode C. elegans. Functionally, the structure serves as an organizer to ventralize tissues in the embryo as well as to guide neuronal positioning and differentiation along the dorsoventral axis of the neural tube.
Floor plate
Cementicle
Blumer's shelf
Bridge 
Terminal web
Lines of Zahn
Aidoc
Enterolith