CHAP. XIX.] DURING UTERINE LIFE. 887 



This bent tube gradually undergoes various modifica- 

 tions, due to the progressive thinning away of its walls 

 in certain places, and to local thickenings (owing to the 

 growth and development of new nerve matter) in other 

 parts. These latter regions of increased thickness corre- 

 spond with future gangKonic centres, which gradually, in 

 the regions already indicated, develop into the Cerebellum, 

 Pons Varolii, Corpora Quadri- 

 gemina, Crura Cerebri, Thalami, 

 and Cerebral Hemispheres with 

 their enclosed Corpora Striata and 

 ' various commissures.' 



From the 7- 9th weeks, the 

 ' middle swelling ' or vesicle (Me- 

 sencephalon), representing the fu- FIG. i24.-verticai section of 



,. , . . , , the Brain of a Human Embryo 



ture Corpora Quadngemma, is the of Fourteen Weeks, magnified 



rQOSt prominent Segment Of the three diameters. (Sharpey, after 

 to Beichert.) c, Cerebral Hemi- 



brain. The Cerebellum even at sphere ; c c, corpus caiiosum be- 



the latter date is represented only 



by a thin lamella Stretching acrOSS third ventricle and the 'pineal 



.,',-, , . , body ' : t h. thalamus opticus ; 3, 



the back of the upper part of the third ventr icie; i, olfactory 



Medulla, While the future Cerebral bulb; eg, corpora quadrlgemina; 



. . cr, crura cerebri, and above 



Hemispheres exist as mere oblong them the 'aqueduct of 



ampulla (fig. 122, 3) projecting ^^i 



downward and forwards from the PV, P ons varoiii; , medulla 



original ' anterior swelling.' From o1 



the under part of this same swelling (Thalamencephalon), 



projects the ' infundibulum,' which either at this time 



or a little later becomes connected with the Pituitary 



Body a structure whose real nature and origin are still 



involved in much obscurity. From about the eighth 



week also the Thalamencephalon is so thinned away above 



(fig. 122, v) that the ' third ventricle ' becomes covered 



only by membrane the ' velum interpositum.' At the 



