PITUITARY BODY—PINEAL BODY. 485 
bridges or commissures, which have considerable classifica- 
tory 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 bears 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 roof 
of the mouth. The two parts unile 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 1s 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 Anxguzs, and Baldwin Spencer securely confirmed this 
in the New Zealand “‘ lizard” (Sphenodox), where the pineal body 
shows distinct traces of a retina. In /etromyzon both the epiphysis 
