THE BRAIN 



343 



The interventricular foramen (of Monro) is at first a wide opening (Fig. 

 343), but is later narrowed to a slit, not by constriction but because its boundaries 

 grow more slowly than the rest of the telencephalon (Fig. 347). 



Diencephalon 



Mesciuiphalmi 



Corpus manuiiilUirc 



J'lilliiim 



Pars ant. olf. lobe 



Tuber ciucr^'uni 



'l^f^Jtm^^ Pars post. olf. lobe 



I nfundibulum Optic stalk 

 Fig. 346. — Lateral view of (he fore-brain and mid-brain of a 13.6 mm. embryo (His). 



X, **** p 



,.Miii««riIf7//fJlnnm>..^ 



Lateral ventricle 



Chorioid plexus of lateral '- 



ventricle 



Thalamus 



Corpus striatum -f 



Third ventricle 



VjlUiLikiitii II ■■"■- 3 



Fig. 347. — ^Transverse section through the fore-brain of a, 16 mm. embryo showing the early develop- 

 ment of the chorioid plexus and fissure (His). 



The third ventricle extends some distance into the caudal end of the telen- 

 cephalon and laterally in this region the optic vesicles develop. Into each optic 

 stalk extends the optic recess (Fig. 343). 



