The structure of the pallial region of the cerebral hemisphere of 20 mm rabbit embryos was studied with aniline and Golgi stains. The presence of a cortical plate distinguishes this stage from 15 mm embryos. It is a dense layer consisting of nerve cells which have migrated from the matrix lamina and represents the layer which willform most of the cerebral cortex. Golgi impregnations at this stage show that it consists largely of pyramidal cells whose apical dendrites ascend and branch in the marginal lamina. Neuroblasts in the matrix and intermediate laminae have distinctive morphology depending on their distance from the ventricular surface: (1) ascending axons arise from the round or elongated somata of cells in the matrix lamina, (2) horizontal axons and multiple short processes arise from neurblast somata in the lower part of the intermediate lamina, and (3) descending axons originate from the inferior side of bipolar neuroblasts in the upper intermediate lamina. A single radial process, the preapex, arises from the other side. This sequence of changes suggests that neuroblast somata rotate around the axons as they migrate through the lower intermediate lamina and that new lengths of axons are "spun out" as neurblasts become located at higher levels. It also raises the possibility that the preapex plays an active role in the process of migration.
