348 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



The fibers of the anterior commissure cross in the lamina terminalis ventral 

 to the hippocampal commissure. They arise in a cranial and a caudal division. 

 The fibers of the former take their origin from the olfactory stalk and the adjacent 

 cortex. The fibers of the caudal division pass ventrally about the corpus striatum, 

 between it and the cortex, and may be derived from one or both of these regions. 



The corpus callosum appears cranial and dorsal to the hippocampal com- 

 missure in the roof of the thickened lamina terminaHs (Fig. 351 A). Through 



Lobus parietalis 



Lateral 

 fissure 



Lobus 

 frontalis 



Lobus 

 temporalis 



Pons 



Lobus 

 occipitalis 



Cerebellum 



Myelenceph- 

 alon 



Spinal cord 



Fig. 3S2. — Lateral view of the brain of a 90 mm. fetus (His). 



its fibers, which arise from neuroblasts in the wall of the neopalHum (cerebral cor- 

 tex), 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 

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

 structure and shows the form wliich is characteristic of the adult (Fig. 351 B). 



