front matter 1

Segregating

ventral horn

motoneurons

Segregating

ventral horn

motoneurons

Settling

ventral horn

interneurons

Cellular roof plate Fibrous roof plate

Floor plateFloor plate

Expanding

dorsal canal

Expanding

dorsal canal

Receding

ventral canal

Receding

ventral canal

Central

canal

Central

canal

Settling

ventral horn

interneurons

Migrating and settling

dorsal horn neurons

Migrating and settling

dorsal horn neurons

Intermediate

interneurons

Intermediate

interneurons

Roof plate

Floor plate

Dorsal root ganglion

Dorsal root

Expanding

gray matter

Expanding

white matter

Lateral plate

(neuroepithelium, NEP cells)

Dorsal root

bifurcation zone

Depleting ventral

interneuron source?

Depleting ventral

interneuron source?

Depleted motoneuron

source?

Depleted motoneuron

source?

Active source of

glial precursors?

Active source of

glial precursors?

Dorsal

neuroepithelium

Dorsal

neuroepithelium

Intermediate

neuroepithelium

Intermediate

neuroepithelium

Ventral

neuroepithelium

Ventral

neuroepithelium

Sojourn zone

of premigratory

neurons destined

for the dorsal horn

Sojourn zone

of premigratory

neurons destined

for the dorsal horn

Dorsal

root

Lateral cervical

nucleus?

axial

motoneurons

axial

motoneurons

proximal upper

limb motoneurons

proximal upper

limb motoneurons

distal

upper limb

motoneurons

distal

upper limb

motoneurons

Incipient dorsal root

collateralization zone

Sulcus

limitans

Sulcus

limitans

Dorsal

funiculus

Spinal canal with expanding

dorsal and receding ventral

components

Ventral

white commissure

Ventral

white commissure

Incipient

ventral median

fissure

V

e

n

tral fu

n

ic

u

l

u

s

V

e

n

tral fu

n

ic

u

l

u

s

L

a

t

e r a l f u n i c u l u s

L

a

t

e r a l f u n i c u l u s

Large arrows extending from the neuro-
epithelium indicate migrating neurons.

PLATE 9B