436 



STRUCTURE OF VERTEBRATA. 



united by bridges or commissures, which have considerable 

 classificatory importance. With the anterior region of the 

 hemispheres olfactory lobes are associated. 



In Cyclostomata, Ganoids, and Teleosteans, the fore Ijrain has no 

 nervous roof, but is covered by a epithelial pallium homologous with 

 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 ner- 

 vous roof, and this persists in higher 

 Vertebrates. 



The optic thalami (thalamen- 

 cephalon, 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 per- 

 sists as the third ventricle of the 

 brain ; the thin roof gives off the 

 dorsal pineal outgrowth or epiphysis, 

 and uniting with the vascular pia 

 mater, or brain membrane, forms 

 a choroid plexus; the lateral walls 

 become much thickened (optic 

 thalami) ; the thin floor gives off 

 a slight ventral evagination, or in- 

 fundibulum, which bears the enig- 

 matical pituitary body or hypo- 

 physis. 



The Pituitary Body. — This is derived 

 in part from the brain and in part from 

 the mouth, and is e.\tremely difiicult to 

 understand. It is apparently equivalent 

 in part to the sub-neural gland of Tuni- 

 cates, but this does not carry us much 

 further. Dohrn connected it with two 

 abortive gill slits, but the evidence seems 

 insufficient. Beard has interpreted it as a 



residuum of the original mouth which Vertebrates are supposed to have 

 possessed before the iiersistent one with which we are familiar was 

 evolved, and of the innervation of that hypothetical structure, but 

 again confirmation seems wanting. Of its physiological nature we 



Fn-,. 139. — Origin of 

 Pineal Body. (After 



BE..\Rn.) 



Lowest figure — a secLion 

 through the first embrvonic 

 vesicle, while the medullar^' 

 groove {g) is still open : c?, 

 optic outgrowths. Middle 

 figure shows beginning of 

 pineal upgrowth (/). Top- 

 most figure shows a later 

 stage. 



