Table 1.

Glossary of structures located at the axial midline of the mouse embryo.

floor plate	Region of developing neural tube at ventral midline composed of cells that are involved in patterning the ventral part of neural tube.
head process	Anterior end of notochord that projects into the head of mammalian and avian embryo and has its origin from cells of the midgastrula organizer (MGO) (Kinder et al., 2001)
midline	Refers to tissues with an axial position within the embryo. The node, notochordal plate, notochord, as well as the neural tube are midline structures.
node	A morphologically-distinct teardrop-shaped concave structure comprised of apically constricted epithelial cells, present at the anterior extremity of the primitive streak. Functionally it contains motile cilia, whose unidirectional rotation is involved in establishing left-right symmetry breaking.
notochord	Transient rod-like mesodermal structure located beneath the neural tube and running the anterior-posterior length of the vertebrate embryo.
notochordal plate	Notochord precursor cells before the formation of the rod like structure
prechordal plate	Anterior-most mesoderm in the vertebrate embryo, located anterior to the notochord, that gives rise to various ventral tissues of the head and has its origin in cells of the early gastrula organizer (EGO) (Kinder et al., 2001)
posterior notochord (PNC)	The indentation at the posterior end of the notochord. The PNC is characterized by features known to be involved in laterality determination. The gastrocoel roof plate (GRP) in Xenopus is equivalent to the mammalian PNC. (Blum et al., 2007)