front matter 1

Roof plateRoof plate

Floor plateFloor plate

VVentral white commissureentral white commissure

Expanding

dorsal

canal

Central

canal

Receding

ventral

canal

Receding

ventral

canal

Settled

ventral horn

motoneurons

Settled

ventral horn

motoneurons

Migrating

ventral horn

motoneurons

and inter-

neurons

Migrating

ventral horn

motoneurons

and inter-

neurons

Migrating and settling

dorsal horn neurons

Migrating and settling

dorsal horn neurons

Intermediate

interneurons

Intermediate

interneurons

Roof plate

Floor plate

Diamond-shaped spinal canal

with expanding dorsal and

receding ventral components

Dorsal root ganglion

Dorsal root

Expanding gray matter

Vascularization reaches

the dorsal neuroepithelium

Expanding white matter

Lateral plate

(neuroepithelium, NEP cells)

Dorsal root

bifurcation zone

Expanding tip

Active ventral

interneuron source?

Active ventral

interneuron source?

Depleted motoneuron

source?

Depleted motoneuron

source?

Active source of

glial precursors?

Active source of

glial precursors?

Crisscrossing

streams of

migrating

neurons

growing

axons ipsi-

laterally

and contra-

laterally

Crisscrossing

streams of

migrating

neurons

growing

axons ipsi-

laterally

and contra-

laterally

Sulcus limitans

Postulated sojourn zone

of premigratory neurons

destined for the

dorsal horn

Ventral

neuroepithelium

Ventral

neuroepithelium

Intermediate

neuroepithelium

Intermediate

neuroepithelium

Dorsal

neuroepithelium

Dorsal

neuroepithelium

Dorsal funiculus

Incipient

ventral median

fissure

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

D

o

r

s

a

l

f

u

n

i

c

u

l

u

s

D

o

r

s

a

l

f

u

n

i

c

u

l

u

s

V

e

n

t

r

al

f un i

cu

l

u

s

V

e

n

t

r

al

f un i

cu

l

u

s

Large arrows
extending from the
neuroepithelium
indicate many
migrating neurons.
Arrows in the spinal
canal indicate the
direction of expan-
sion or contraction.

PLATE 7B