Rotation of the Hemisphere
The hemispheric vesicle does not expand evenly in all directions during its develop-ment but widens primarily in caudal and basal directions. The temporal lobe is formed in this way, and it finally turns ros-trally in a circular movement (C); to a lesser degree, such a rotation can also be observed with the frontal lobe. The axis around which the hemispheric vesicle rotates is the insu-lar region; like the putamen (E6) lying beneath it, the insula does not participate in the movement. Other structures of the hemisphere, however, follow the rotation and end up having an arched shape in the mature brain. The lateral ventricle (D7) forms such an arch with its anterior and in-ferior horns. The lateral portion of the stri-atum, the caudate nucleus (E8), participates in the rotation as well and follows precisely the arched shape of the lateral ventricle. The main part of the archipallium, the hippo-campus (F9), moves from its original dorsalposition in basal direction and comes to lie in the temporal lobe. The remnants of the archipallium on the dorsal aspect of the cor-pus callosum, the indusium griseum (F10), and the fornix (F11) reflect the arched ex-pansion of the archipallium. The corpus cal-losum (F12) also expands in caudal directionbut follows the rotation only partially as it develops only late toward the end of this process.
D13 Third ventricle.