THE BRAIN 



349 



occipital and parietal lobes (Fig. 33S) ; (3) the calcarinc fissure which includes 

 between it and the parieto-occipital fissure the cuneus and marks the position of 

 the visual area of the cerebrum; (4) the collateral fissure on the ventral surface 

 of the temporal lobe, which produces the inward bulging on the floor of the 

 posterior horn of the ventricle known as the collateral eminence. The calcarine 

 fissure also affects the internal wall of the ventricle, causing the convexity 

 termed the calcar avis. 



Simultaneously with the development of the collateral fissure appear other 

 shallower depressions known as sulci. These have a definite arrangement and 



Gyrus cinguli 



Corp. callosum Siilc. Corp. callosi 



Spknium 



Parieto-occipital fissure 



Space of 

 septum 

 pellu- 

 cidum 

 Rostral 

 lamina 

 Parol- 

 factory 

 area 



Cuneus 



Fig. 33S.- 



Olfactory lobe Fissura rhinica 



Optic nerve Temporal lobe 



-Median surface of the right cerebral hemisphere from a seven months' fetus (Kollmann). 



with the fissures mark off from each other the various functional areas of the cere- 

 brum. The surface convolutions between the depressions constitute the gyri 

 and lobules of the adult cerebrum. 



Histogenesis of the Cerebral Cortex.— The three primitive zones typical 

 of the neural tube are differentiated in the wall of the pallium: the ependymal, 

 mantle and marginal layers. During the first two months the cortex remains 

 thin and differentiation is slow. At eight weeks neuroblasts migrate from the 

 ependymal and mantle zones into the marginal zone and give rise to layers of 

 pyramidal cells typical of the cerebrum. The differentiation of these layers is 

 most active during the third and fourth months. From the fourth month on the 



