front matter 1

A

B

CR 19.1 mm,CR 19.1 mm, C8965 C8965

CR 22.0 mm, C8553

1 mm

1 mm

Floor

plate

Roof

plate

Undivided

neuroepi-

thelium

Central

canal

Gray

matter

Subdivided

neuroepithelium

Dorsal

Intermediate

Ventral

Subdivided

gray matter

Dorsal

horn Intermediate

gray Ventral

horn













Figure 5. Enlarged views of the CR 19.1 mm (A) and CR 22 mm (B) specimens (note scale differences). The top views show only the surface features
of the neuroepithelium on the right side; bottom views show the adjacent gray matter on the right side. In A, the ventral neuroepithelium has receded
but the intermediate and dorsal neuroepithelia are still active (now with many undulations, downward arrowheads). The outer surface of the posterior
ventral horn has bulges and depressions (upward arrowheads) representing the segregation of the ventral motoneurons into columns in the cervical
enlargement. In B, the same features are present, but now the intermediate neuroepithelium is starting to recede, and the dorsal horn is growing rapidly.

FIGURE 5 Neuroepithelium and Gray Matter – GW7.0 to GW7.2