440 



STRUCTURE OF VERTEBRATA. 



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 out- 

 growths, 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 pia mater, or vascular brain 

 membrane, forms a choroid plexus ; 

 the lateral walls become much thick- 

 ened (optic thalami) ; the thin floor 

 gives off a slight ventral evagination, 

 or infundibulum, which bears the en- 

 y*< igmatical pituitary body or hypophysis. 



The pituitary 'body. — This is derived in 

 part from the brain and in part from the 

 mouth, and is extremely difficult to under- 

 stand. It is apparently equivalent in part to 

 the sub-neural gland of Tunicates, but this 

 does not carry us much further. Dohrn con- 

 nected it with two abortive gill-slits, but the 

 evidence seems insufficient. Beard has inter- 

 preted it as a residue of the original mouth 

 which Vertebrates are supposed to have pos- 

 sessed before the persistent one with which we 

 are familiar was evolved, and of the in- 

 nervation of that hypothetical structure ; but 

 again confirmation seems wanting. Of its 

 physiological nature we know almost nothing, 

 beyond that a pathological state of this organ 

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

 acromegaly. 

 The pineal body.— The dorsal upgrowth 

 (or epiphysis) from the roof of the thalamencephalon is represented, 

 though to a varying extent, in all Vertebrates. It is terminally differ- 

 entiated 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. 



Fig. 188. — Origin of 

 pineal body. — After 

 Beard. 



Lowest figure — a section 

 through the first embry- 

 onic vesicle, while the 

 medullary groove (£•.) is 

 still open ; o., optic out- 

 growths. Middle figure 

 shows beginning of pin- 

 eal outgrowth (p.). Top- 

 most figure shows a later 

 stage. 



