THE BRAIN 



341 



tissue and remains connected to the diencephalbn by a permanent in- 

 fundibular stalk (Fig. 147). The anterior lobe and the pars intermedia 

 elaborate important internal secretions. 



Caudal to the infundibulum, in the floor plate, are developed in 

 order the tuber cinereum and the mammillary recess (Figs. 341, 344 and 

 346). 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 com- 

 pressed 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. 



Mesencephaton Diencephalon Pallium 



Mammillary body 



Hypophysis 



Optic stalk Lobus olfactorius 



FIG. 344. 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. 344). The primitive structures of the neural tube 

 can no longer be recognized, but the telencephalon is regarded as repre- 

 senting greatly expanded alar plates and is, therefore, essentially a paired 

 structure. Each of the paired outgrowths expands cranially, dorsally, 

 and caudally, and eventually overlies the rest of the brain (Figs. 344, 345 

 and 346). 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 develop- 

 ment and thus there is formed the great longitudinal fissure between the 

 hemispheres. The lamina is continuous caudally with the roof plate of 



