front matter 1

Roof plateRoof plate

Floor plateFloor plate

Dorsal

neuroepithelium

(alar plate)

Intermediate

neuroepithelium

Ventral

neuroepithelium

(basal plate)

Central canal

Expanding

ventral canal

Dorsal canal

Earliest

motoneurons

Primordial

white matter

Primordial

white matter

Earliest neurons in

dorsal horn (lamina I?)

Earliest

commissural

neurons?

Roof plate

Floor plate

Notochord

Slit-shaped

spinal canal

expanding

ventrally

Somite

Primordial white Dorsal root ganglion

matter

Primordial gray

matter

Lateral plate (neuroepi-

thelium, NEP cells)

LATERAL PLATE

Approximate

location of the

future sulcus limitans

(see Plate 4)

Large arrows extending from the neuroepithelium
indicate migrating neurons into the primordial gray
matter. Progressively smaller arrows extending
from the neuroepithelium indicate progressively
fewer migrating neurons. Small arrow in the spinal
canal indicates the direction of expansion.

PLATE 3B