THE BRAIN 341 



dent with the differentiation of the anterior lobe the infundibular anlage of the 

 posterior lobe loses its cavity, but the walls of the infundibulum persist as its 

 sohd, permanent stalk. The lobe enlarges and its cells are differentiated into a 

 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. 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 compressed until it forms 



Mesencephalon Diencephalon Pallium 





il" 



1 



I 



optic stalk Lobus olfactoritis 



Fig. 344. — Lateral view of the fore- and mid-brains of a 10.2 mm. embryo (His). 



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. 



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 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. 344, 345 and 346). The telencephalon is differentiated 

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



