front matter 1

Roof plateRoof plate

Floor plateFloor plate

Dorsal

neuroepithelium

Edge of gray

matter

Intermediate

neuroepithelium

Ventral

neuroepithelium

Ventral root

Incipient

ventral

funiculus

Central

canal

Dorsal

canal

Ventral

canal

Settling

v

Settlingentral hornmotoneurons

ventral hornmotoneurons

Migrating

ventral horn

motoneurons

and interneurons

Earliest dorsal horn neurons

(laminae I and IV)

Intermediate

interneurons

Primordial

white matter

Roof plate

Floor plate

Diamond-shaped

spinal canal

expanding in

dorsal, lateral, and

ventral directions

Expanding dorsal

root ganglion

Expanding gray matter

Expanding white

matter

Lateral plate (neuroepi-

thelium, NEP cells)

LATERAL PLATE

Oval bundle of His (dorsal

root bifurcation zone)

Sulcus limitans

Incipient dorsal

funiculus

Large arrows extending from the neuroepithelium indicate many migrating neurons. Progressively
smaller arrows extending from the neuroepithelium indicate progressively fewer migrating neurons.
Arrows in the spinal canal indicate the direction of expansion.

PLATE 4B