348 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



The corpus callosum appears cranial and dorsal to the hippocampal 

 commissure in the roof of the thickened lamina terminalis (Fig. 351 A). 

 Through its fibers, which arise from neuroblasts in the wall of the neo- 

 pallium (cerebral cortex), nearly all regions of one hemisphere are asso- 

 ciated with corresponding regions of the other. With the expansion of 

 the pallium, the corpus callosum is extended cranially and caudally by 

 the development of interstitial fibers. The fibers first found in the corpus 



Lobus parietalis 



Lateral 

 fissure 



Lobus 

 fronialis 



Lobus 

 temporalis 



Pans 



Lobus 

 occipitalis 



Cerebellum 



Myelenceph- 

 alon 



Spinal cord 



FIG. 352. Lateral view of the brain of a 90 mm. fetus (His). 



callosum arise in the median wall of the hemispheres. In fetuses of five 

 months this great commissure is a conspicuous structure and shows the 

 form which is characteristic of the adult (Fig. 351 B). 



Form of the Cerebral Hemispheres. When the telencephalon ex- 

 pands cranially, caudally, and at the same time ventrally, four lobes may 

 be distinguished (Fig. 352): (i) a cranial frontal lobe; (2) a dorsal parietal 

 lobe; (3) a caudal occipital lobe; and (4) a ventro-lateral temporal lobe. 

 The ventricle extends into each of these regions and forms respectively 



