P1TUITAR Y BOD Y PINEAL BOD Y. 485 



bridges or cpmmissures, which have considerable classifies 

 lory importance. With the anterior region of the hemi- 

 spheres olfactory lobes are associated. 



In Cyclostomata, " Ganoids," and Teleosteans, the fore-brain has no 

 nervous roof, but is covered by an epithelial pallium which resembles 

 what is called the choroid plexus of the third ventricle in higher Verte- 

 brates. This choroid plexus is a thin epithelium, with blood vessels in 

 it. But in Elasmobranchs, Dipnoi, and Amphibians the basal parts of 

 the fore-brain have grown upwards to form a nervous roof, and this 

 persists in higher Vertebrates. 



The optic thalami (thalamencephalon or tween-brain) 

 form the second region of the adult brain. Hence arise 

 the optic outgrowths, which form the optic nerves and 

 some of the most essential parts of the eyes. The 

 original cavity persists as the third ventricle of the brain ; 

 the thin roof gives off the dorsal pineal outgrowth or epi- 

 physis, and, uniting with the pia mater, or vascular brain 

 membrane, forms a choroid plexus; the lateral walls 

 become much thickened (optic thalami); the thin floor 

 gives off a slight ventral evagination, or infundibulum, 

 which bears the enigmatical pituitary body or hypo- 

 physis. The infundibulum also Dears in most Teleosts 

 a peculiar posterior saccus vasculosus, which seems 

 to be a sense organ. It is not developed except in 

 Fishes. 



The pituitary body. This is derived partly from a downgrowth 

 from the thalamencephalon and partly from an upgrowth from the rooi 

 of the mouth. The two parts unite to form a complex little organ, 

 whose morphological nature is very puzzling. It produces an internal 

 secretion of importance, and a pathological state of the organ is 

 associated in man with certain diseases, e.g. acromegaly. 



. The pineal "body. The dorsal upgrowth from the roof of the 

 thalamencephalon is represented, though to a varying extent, in all 

 Vertebrates. It consists of two parts, a pineal organ or epiphysis 

 proper, and a parietal organ, which arises as a rule from the epiphysis 

 but may have an independent origin in front of it. It is probable that 

 they were originally right and left members of a pair. The parietal 

 organ may become atrophied, but in some cases, especially in Reptiles, 

 it is terminally differentiated into a little body known as the pineal 

 body. This was entirely an enigma until De Graaf discovered its eye- 

 like structure in Anguis, and Baldwin Spencer securely confirmed this 

 in the New Zealand "lizard" (Sphenodon\ where the pineal body 

 shows distinct traces of a retina. In Petromyzon both the epiphysis 



