Chap. XIX.] DURING UTERINE LIFE. 337 



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 ganglionic 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- 9 th weeks, the 



* middle swelling ' or vesicle (Me- 

 sencephalon), representing the fu- fig. 124.— vertical section of 



. /->, /-^ T • • • J 1 the Brain of a Human Embryo 



ture Corpora Qaadrigemma, is the of Fourteen weeks, magnified 

 most prominent segment of the thi-ee diameters. (Sharpey, after 



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

 the latter date is represented only gtr^r— ^^it" 



by a thin lamella stretching across third ventricle and the 'plneal 

 . T T 1 p i 1 1 /. , 1 body ' : th, thalamus opticus ; 3, 



the back of the upper part of the third ventricle; I, olfactory 

 Medulla, while the future Cerebral ^"l^^ eg. corpora quadrlgemlna; 

 . . 11*^''' crura cerebri, and above 



Hemispheres exist as mere oblong them the 'aqueduct of syMus- 

 ampulte (fig. 122, 3) projecting ^::^^;^-::T^,^ 

 downward and forwards from the pv, poi.s varoiu; m, meduiia 

 original ' anterior swelling.' From ""'""S^"^^^- 

 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 



