XII.] THE THALAMENCEPHALON. 373 



In the later stages of development the unchanged 

 portion of the infundibulum becomes gradually pro- 

 longed and forms an elongated diverticulum of the 

 third ventricle, the apex of which is in contact with 

 the pituitary body (Fig. 120 hph). 



The posterior part of the primitive infundibulum becomes the 

 corpus albicans, which is double in Man and the higher Apes ; 

 the ventral part of the posterior wall forms the tuber cinereum. 

 Laterally, at the junction of the optic thalami and infundibulum, 

 there are continued some of the fibres of the crura cerebri, which 

 are probably derived from the walls of the infundibulum. 



The sides of the thalamencephalon become very 

 early thickened to form the optic thalami, which con- 

 stitute the most important section of the thalamen- 

 cephalon. These are separated on their inner aspect 

 from the infundibular region by a somewhat S-shaped 

 groove, known as the sulcus of Monro, which ends in 

 the foramen of Monro. They also become secondarily 

 united by a transverse commissure, the grey or middle 

 commissure, which passes across the cavity of the third 

 ventricle. 



The roof undergoes more complicated changes. It 

 becomes divided, on the appearance of the pineal gland 

 as a small papilliform outgrowth (the development of 

 which is dealt with below), into two regions a longer 

 anterior in front of the pineal gland, and a shorter pos- 

 terior. The anterior region becomes at an early period 

 excessively thin, and at a later period, when the roof of 

 the thalamencephalon is shortened by the approach of 

 the cerebral hemispheres to the mid-brain, it becomes 

 (vide Fig. 120 chd 3) considerably folded, while at the 

 same time a vascular plexus is formed in the pia mater 



