THE NERVOUS SYSTEM. 561 



of Reil. In the ape the frontal operculum is absent and the island of Reil 

 partly exposed. 



Toward the end of the third month the calcarine fissure appears, producing 

 on the ventricular surface the eminence known as the calcar avis. At the 

 beginning of the fourth month the parieto-occipital fissure unites with it forming 

 the cuneus. The parieto-occipital reaches the superior border of the hemi- 

 spheres by the sixth or seventh month. At the sixth month the fissure of Rolando 

 (central fissure) appears. The condition of the surface of the hemisphere at 

 the end of the seventh month is shown in Figs. 485 to 488. 



The early histogenetic development of the pallial wall, resulting in the dif- 

 ferentiation into the usual ependymal, mantle and marginal layers, has been 

 mentioned. (Fig. 489). The next stage, already alluded to (p. 549), marks a 



Gyrus front, med. v ^ 



j^. 1^^ I j^k Sulcus front, sup. 



Gyrus front, inf. /jjjH ^Bt \ 



Dk Sulcus front, inf. 

 Gyrus front, sup. \ 



|B& Sulcus prsecentralis 

 Gyrus prsecent. ^Rj * 



Sulcus centralis 

 : ; x . ' , .t. , , 



IHf Sulcus postcentralis 

 Lobulus par. sup. J 



Sulcus interparietalis 



Lobulus par. inf. 



^^r Fissura parieto-occipit. 



Lobus cccipitalis 



FIG. 486. Dorsal view of the cerebral hemispheres of a human foetus at the end 

 of the yth month. Kollmann. 



difference in development between the pallium, as well as other supraseg- 

 mental structures, and the rest of the walls of the neural tube. This stage 

 consists apparently in a further migration outward of the neuroblasts and their 

 accumulation under the marginal layer, forming, at eight weeks, a definite 

 layer of closely packed cells, the beginning of the cortex (Fig. 490). Later 

 neuroblast migrations probably add to this layer. It has already been men- 

 tioned that the fibers of the thalamic radiation appear in the pallial walls about 

 this time. They proceed internally to the cortical layer and thus mark the 

 beginning of the fiber layer (medullary layer) which by later myelination 

 becomes the white matter of the hemispheres. 



The extension of the process of differentiation of the cortical layer from the 

 region of the corpus striatum over the rest of the pallium has also been men- 

 tioned (p. 549). It is probable that the afferent pallial fibers (thalamic radia- 

 tion) in their growth keep pace with this process. Those fibers from the lateral 



