front matter 1

71%71%

7%

3%4%

15%

G

W

4

. 0

65%65%

7%

6%

2% 2%

18%

G

W

4

. 5

47%47%

17%

20.5%

1.5%2%

12%

G

W

4

. 7
5

28%28%

13%

38%

1% 1%

19%

G

W

5

. 5

G

W

(^6).
5

21%21%

7%

46%

1.5% .5%

24%

2.6% 2.8%

53%

.6% .3%

40.7%

G

W

8

. 5

FLOOR PLATE

WHITE MATTER SPINAL CANAL ROOF PLATE

GRAY MATTER NEUROEPITHELIUM

Figure 34. A series of pie-shaped graphs showing the proportional areas of the white matter (white), gray matter (gray), spinal canal (light green),
neuroepithelium (dark green), roof plate (orange), and fl oor plate (red) at the cervical enlargements of specimens from gestational week (GW) 4.0
(upper right graph) to GW8.5 (lower right graph). The section measured from the GW4 specimen is illustrated in Plate 2, GW4.5 in Plate 3, GW4.75
in Plate 4, GW5.5 in Plate 7, GW6.5 in Plate 8, and GW8.5 in Plate 12. The neuroepithelium dominates from GW4-4.75 when stem cells are rapidly
increasing in preparation for successive waves of neurogenesis. Even though the neuroepithelium declines in its proportion throughout the entire period,
it is actually increasing its absolute area up to GW6.5 (see Figure 32). In contrast, the peak proportional areas of the spinal canal (GW4.75-5.5) occur
at the same time as the largest absolute area and their decline at GW6.5-8.5 is associated with dramatic shrinkage of the neuroepithelium (Figure 32).
When neurons are produced and migrate out of the neuroepithelium, the area of the gray matter proportionally increases. The fi rst proportional increase
between GW4.75-5.5 is almost exclusively due to growth of the ventral horn. Later proportional increases are due to continual growth of the ventral
horn, the emerging intermediate gray, and the rapid expansion of the dorsal horn. The white matter measured at GW4.0-4.75 is primordial and contains
only a few axons; that area is the same as the mantle layer described in traditional developmental studies. From GW5.5 onward, the proportional area
increases are due to accumulating axons, mainly in the dorsal funiculus, ventral funiculus, and ventral commissure at GW5.5 and GW6.5. By GW8.5
the proportional area of the white matter sharply increases. The dorsal funiculus greatly expands when long ascending collaterals form the gracile and
cuneate fasciculi. The roof and fl oor plates have their largest proportional areas early (GW4.0-4.75) supporting the animal experimental evidence of a
promint role in the overall dorsal–to–ventral differentiation of the gray matter and growth of axons parallel to the dorsal midline and across the ventral
midline (see Chapter 4 in Altman and Bayer, 2001).

FIGURE 34 Proportional Areas at the Cervical Enlargement