516 THE HUMAN EMBRYO. 



The changes during the last three months of faital life con- 

 sist chiefly in the formation of the fissures of the cerebral 

 hemispheres and cerebellum, and in the gradually increasing 

 complication of all parts of the brain. 



b. The Cerebral Hemispheres. 



The unpaired vesicle of the hemispheres is already present 

 on the fifteenth day, i.e. at the time when closure of the brain tube 

 is effected. Towards the close of the fourth week, a median 

 dorsal ridge appears along the roof and anterior wall of the un- 

 paired vesicle, and then becomes folded down so as to project as 

 a septum into the cavity of the vesicle, which it partially divides 

 into the right and left hemispheres. 



During the fifth week, a sheet of vascular connective tissue 

 grows into the cleft between the hemispheres, and forms the 

 basis of the falx cerebri. Before the end of the week the inner 

 or mesial walls of the hemispheres, bordering the falx cerebri, 

 become thrown into folds which project into the cavities of the 

 hemispheres ; blood-vessels from the falx cerebri soon grow in 

 between the two layers of these folds, and give rise to the 

 choroid plexuses of the lateral ventricles. 



Up to the end of the fifth week there is no marked difference 

 in the thickness of the walls of the hemispheres at different places, 

 but from this time growth takes place very unequally in different 

 directions, some parts thickening very rapidly, while others be- 

 come reduced to single layers of epithelial cells. 



The first important thickenings to appear are those which 

 give rise to the corpora striata. These arise, early in the fifth 

 week, as a pair of ridge-like thickenings of the ventral walls 

 of the hemispheres, which project into the lateral ventricles and 

 form prominent lower lips to the foramina of Monro, through 

 which these ventricles communicate with the third ventricle or 

 ca\ity of the thalamencephalon. 



The corpora striata are formed in part as actual thickenings 

 of the walls of the hemispheres ; but their appearance is due in 

 the first instance to folding of the whole thickness of their walls ; 

 the depressions funned by the Sylvian fissures on the outer 

 surfaces of the hemispheres corresponding to the inwardly project- 

 ing ridges of the corpora striata. It is sometimes stated that 

 the corpora striata are formed by the depressions of the surface 



