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TEXT-BOOK OF EMBRYOLOGY. 



growing more rapidly but later the fronto-parietal, thereby changing the 

 direction of the Sylvian fissure from an oblique to the more horizontal angle 

 characteristic of man as compared with the ape. In the meanwhile the 

 development of the frontal lobe leads to its also overlapping the insula. If the 



Parietal lobe 



Occipital 



Mesencephalon 

 Cerebellum 



Frontal lobe 

 Insula 



Bulbus olfactorius 



\ \ > Gyrus olfactor. lat. 



\ Gyrus semilunaris 

 Gyrus ambiens 



FIG. 484. Lateral view of the brain of a human foetus at the beginning of the 

 4th month. Kollmann. 



frontal lobe fully develops, it forms a U-shaped operculum between the fronto- 

 parietal and the orbital, if it does not so fully develop it forms a V-shaped 

 operculum, and a still less developed condition is shown by a Y-shaped arrange- 

 ment in which the frontal lobe does not completely separate the fronto-parietal 



Sulcus corp. callosi 

 Corpus callosum | Splenium 



Gyrus cinguli | | Fissura parieto-occip. 



Cavum septi pellucid) 



Lamina rostralis 



Area parolfactoria 



(praeterminalis) 



fe Cuneus 



Fissura calcarina 



N. olfact. | | Fiss. rhinica 

 N. optic. Lob. temp. 



FIG. 485. Median view of the left half of the brain of a human foetus at the end 

 of the yth month. Kollmann. 



and temporal opercula. The orbital and frontal opercula are not in apposition 

 till the first year. Conditions of arrested development are thus indicated by 

 the Y-shaped anterior ascending branch of the Sylvian fissure coupled with an 

 absence of the pars triangularis and also by a partial exposure of the island 



