346 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



missure in the roof of the thickened lamina terminalis (Fig. 333 A). Its fibers 

 arise from neuroblasts in the wall of the neopallium (cerebral cortex) and by them 

 nearly all regions of one hemisphere are associated 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 callosum arise in the median wall of the hemispheres. In 



Lateral 

 fissure 



Lobus 

 frontalis 



Lobus 

 temporalis 



Pons 



Lobus parietalis 



Spinal cord 



Fig. 334. — Lateral view of the brain of a 90 mm. embryo (His). 



fetuses of 150 mm. (five months) this great commissure is a conspicuous structure 

 and shows the form which is characteristic of the adult (Fig. ^^,^ B). 



The Form of the Cerebral Hemispheres. — When the telencephalon expands 

 cranially, caudally and at the same time ventrally, four lobes may be distin- 

 guished (1) a cranial frontal lobe; (2) a dorsal parietal lobe; (3) a caudal occipi- 

 tal lobe, and (4) a ventro-lateral temporal lobe (Fig. 334). The ventricle extends 

 into these regions and in each forms respectively the anterior horn, the body, the 

 posterior horn and the inferior horn of the lateral ventricle. The surface extent 



