front matter 1

Expanding tip

Roof plate

Floor plateFloor plate

Dorsal neuroepithelium

Intermediate neuroepithelium

Ventral neuroepithelium compartments:

Ventral white commissure (decussation of commissural axons)

VVentral funiculusentral funiculus

Dorsal canal (expanding rapidly)

Central canal Ventral canal (expansion ending, see Plate 7B)

Settled ventral horn motoneurons

Migrating
ventral horn
motoneurons
and inter-
neurons

Dorsally advancing vascularization

Migrating and settling early dorsal horn neurons (laminae I and IV)

Intermediate
interneurons

Primordial
white matter

Roof plate

Floor plate

Dorsal root ganglion

Dorsal root (nerve + boundary cap)

Ventral rootlets Spinal nerve

Expanding gray matter

Expanding white matter

Ventral horn

Lateral plate (neuroepithelium, NEP cells)

Oval bundle of His (dorsal root bifurca- tion zone)

Dorsal root

Expanding tip

Active ventral interneuron source?

Depleted motoneuron source?

Sojourn zone of late motoneurons and interneurons?

Active source of glial precursors?

Sulcus limitansSulcus limitans

Dorsal funiculus:

Diamond-shaped spinal canal expanding dorsally and ventrally

Large arrows extending from the neuroepithelium indicate migrating neurons. Arrows in the spinal canal indicate the direction of expansion.

PLATE 6B