THE BRAIN 



339 



diffuse tissue resembling neuroglia. About the two lobes of the hypophysis the 

 surrounding mesenchyme develops a connective tissue capsule. 



Caudal to the infundibulum in the floor plate are developed in order the tuber 

 cinereum and the mammillary recess (Figs. 323 and 324). The lateral walls of 

 the latter thicken and give rise to the paired mammillary bodies. 



The third ventricle lies largely in the diencephalon and is at first relatively 

 broad. Owing to the thickening of its lateral walls it is compressed until it forms 

 a narrow vertical cleft. In a majority of adults the thalami are approximated, 

 fuse and form the massa intermedia or commissura mollis, which is encircled by 

 the cavity of the ventricle. 



Mesencephalon 



Diencephalon 



Pallium 



Mammillary body 



Hypophysis 



Optic stalk 



Lobus olfactorins 



Fig. 326. — Lateral view of the fore- and mid-brains of a 10.2 mm. embryo (His). 



The Telencephalon.— This is the most highly differentiated division of the 

 brain (Fig. 326). The primitive structures of the neural tube can no longer be 

 recognized but the telencephalon is regarded as representing greatly expanded 

 alar plates and is therefore essentially a paired structure. Each of the paired out- 

 growths expands cranially, dorsally, and caudally, and eventually overlies the 

 rest of the brain (Figs. 326, 327 and 328). The telencephalon is differentiated 

 into the corpus striatum, rhinencephalon, and pallium (primitive cortex of cerebral 

 hemisphere). The median lamina between the hemispheres lags behind in its 

 development and thus is formed the great longitudinal fissure between the hemi- 

 spheres. The lamina is continuous caudally with the roof plate of the dien- 



