XII.] THE PINEAL GLAND. 375 



above it. On the accomplishment of these changes it 

 is known as the tela choroidea of the third ventricle. 



In the roof of the third ventricle behind the pineal 

 gland there appear transverse commissural fibres, form- 

 ing a structure known as the posterior commissure, 

 which connects together the two optic thalami. 



The most remarkable organ in the roof of the thala- 

 mencephalon is the pineal gland, which is developed as 

 a hollow papilliform outgrowth of the roof, and is at 

 first composed of cells similar to those of the other 

 parts of the central nervous system (Fig. 120 pin). It 

 is directed backwards over the hinder portion of the 

 roof of the thalamencephalon. 



In Birds (p. 116) the primitive outgrowth to form 

 the pineal gland becomes deeply indented by vascular 

 connective-tissue ingrowths, so that it assumes a den- 

 dritic structure (Fig. 119 pin). The proximal extremity 

 attached to the roof of the thalamencephalon soon 

 becomes solid and forms a special section, known as 

 the infra-pineal process. The central lumen of the 

 free part of the gland finally atrophies, but the branches 

 still remain hollow. The infra-pineal process becomes 

 reduced to a narrow stalk, connecting the branched 

 portion of the body with the brain. 



In Mammalia the development of the pineal gland 

 is generally similar to that of Birds, The original out- 

 growth becomes branched, but the follicles or lobes to 

 which the branching gives rise eventually become solid 

 (Fig. 120 pin). An infra-pineal process is developed 

 comparatively late, and is not sharply separated from 

 the roof of the brain. 



No satisfactory suggestions have yet been offered as 



