ABSTRACT
Figure 2. The neuroepithelium, gray matter, and the outer edge of the spinal cord is visible in all eight specimens. The specimens are not shown to scale because structures in the smaller ones (A-D) cannot be seen clearly after reduction to the scale of the largest one (H). At CR 3.3 mm (A), there is no gray matter. The ventral neuroepithelium is larger than the intermediate or dorsal neuroepithelia. At CR 4.0 mm (B) a thin sliver of gray matter, the ventral horn, appears adjacent to the ventral neuroepithelium. At CR 8.0 mm (C), there is gray matter adjacent to all parts of the neuroepithelium, but the ventral horn is largest. The intermediate and dorsal components of the neuroepithelium are growing larger. At CR 10.5 mm (D), the ventral neuroepithelium starts to recede, but the intermediate and dorsal components are still large. The ventral horn is still the largest gray matter component, but neurons are also accumulating in the intermediate gray and dorsal horn. That same process continues at CR 19.1 mm (E). The intermediate neuroepithelium is receding at CR 22.0 mm (F) but the dorsal neuroepithelium is still prominent. The dorsal horn is now larger than the intermediate gray. By CR 36 mm (G), the dorsal neuroepithelium considerably recedes and the dorsal horn is as large as the ventral horn. By CR 56 mm (H), the neuroepithelium has been replaced by an ependyma surrounding the shrinking central canal, and the dorsal and ventral horns are prominent components of the gray matter.
