The radial glia cells, which are used by young neurons as a guidance structure, are aligned perpendicular to the ventricular and pial surfaces. Due to this, the young neurons migrate in radial or vertical directions. This is true for the majority of neurons, especially for the projection neurons. It has been found that some interneurons, particularly GABAergic cells, migrate from the lateral and medial parts of the ventricular eminence towards the cortex in a tangential direction.
The terminal lips of the rostral neuropore fuse together and form the lamina terminalis of the telencephalon. The commissural plate, which follows immediately after in a cranial direction, is used as a guiding structure for the commissural fibres of the telencephalon. At S23, the anterior commissure is the first commissure to develop. It is followed at week 11 by the commissure of the fornix (hippocampal commissure) and at week 12 by the fibres of the corpus callosum.