434 THE PINEAL GLAND. 



hemispheres (fig. 254/w). Its distal extremity dilates somewhat, 

 and in the adult the whole organ forms (Ehlers, No. 337) an 

 elongated tube, enlarged at its free extremity, and opening at 

 its base into the brain. The enlarged extremity may either be 

 lodged in a cavity in the cartilage of the cranium (Acanthias), or 

 be placed outside the cranium (Raja). 



In Petromyzon its form is very different. It arises (fig. 

 2 53 P n ) as a sack-like diverticulum of the thalamencephalon 

 extending at first both backwardsand forwards. In the Ammo- 

 ccete the walls of this sack are deeply infolded. 



The embryonic form of the pineal gland in Amphibia is very 

 much like that which remains permanent in Elasmobranchii ; 

 the stalk connecting the enlarged terminal portion with the 

 brain soon however becomes solid and very thin except at its 

 proximal extremity. The enlarged portion also becomes solid, 

 and is placed in the adult externally to the skull, where it forms 

 a mass originally described by Stieda as the cerebral gland. 



In Birds the primitive outgrowth to form the pineal gland 

 becomes, according to Mihalkovics, deeply indented by vascular 

 connective tissue ingrowths, so that it assumes a dendritic 

 structure (fig. 250 pin). 



The proximal extremity attached to the roof of the thala- 

 mencephalon 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. The branched 

 terminal portion and the stalk obviously correspond with the 

 vesicle and distal part of the stalk of the types already described. 

 In Mammalia the development of the pineal gland is, according 

 to Mihalkovics, generally similar to that of Birds. The original 

 outgrowth becomes branched, but the follicles or lobes to which 

 the branching gives rise eventually become solid (fig. 255 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 to the 

 nature of the pineal gland, unless the view of Gotte be regarded 

 as such. It appears to possess in all forms an epithelial structure, 

 but, except at the base of the stalk (infra-pineal process) in 



