THE BRAIN 



347 



of the cerebral wall, the thin gray cortex, increases more rapidly than the un- 

 derlying white medullary layer. As a result the cortex is folded, producing con- 

 volutions between which are depressions, the sulci and fissures. The chorioidal 

 fissure is formed, as we have seen, by the ingrowth of the chorioid plexus. 

 During the third month the hippocampal fissure develops as a curved infolding 

 along the median wall of the temporal lobe. The infolded cortex forms the 

 hippocampus. The lateral fissure (of Sylvius) makes its appearance also in the 

 third month, but its development is not completed until after birth. The cortex 

 overlying the corpus striatum laterally develops more slowly than the surrounding 



Corpora 

 quadrigemiiia 



Hemisphere of 

 cerebellum 



Vermis ecrebe/ii 



Occipital lobe of 

 cerebrum 



Impression of 

 thalamus 



Temporal lobe 



Lateral recess of 

 ventricle 4 



Fasciculus gracilis 

 Medulla oblongata 



Fig. 335. — Posterior view of the brain from a 100 mm. embryo (Kollmann's Handallas). 



areas and is thus gradually overgrown by folds of the parietal and frontal lobes 

 (fronto-parietal operculum) and of the temporal lobe (temporal operculum). 

 The area thus overgrown is the insula (island of Reil) and the depression so formed 

 is the lateral fissure (of Sylvius). Later, frontal and orbital opercula are developed 

 ventro-laterally from the frontal lobe (Fig. 337). These are not approximated 

 over the insula until after birth. The frontal operculum is included between the 

 anterior limbs of the Sylvian fissure and the extent of its development, which is 

 variable, determines the form of these limbs. 



In fetuses of six to seven months four other depressions appear which later 

 form important landmarks in the cerebral topography. These are: (i) the 



