THE BRAIN 155 



parts developed in the roof plate of the primitive fore-brain. At the 

 junction of cerebral hemispheres and 'twixt-brain (fig. 158) there is an 

 internal epithelial fold, the velum transversum, depending from the 

 cerebral roof. In front of this an outgrowth, the paraphysis, arises 

 on the top of the brain in nearly all vertebrates (figs. 158, 160). 

 It is non-nervous and apparently is an extraventricular chorioid 

 plexus with secretory functions. The other epiphysial structures be- 

 long to the 'twixt-brain and consist of a parietal organ and a pinealis. 

 Both arise from the roof between the habenular gangUon and the 

 posterior commissure, at the boundary between 'twixt- and mid- 

 brains, sometimes as two distinct structures (fig. 161), sometimes as 



Fig, 161. — Section through brain of an Acanthias embryo, 15 mm. long, showing the 

 early appearance of two epiphysial structures, the parietal organ {p) wWch is already 

 degenerating, and the pinealis (c), h, hypophysis, growing in from the oral epithelium; 

 », infundibulum; n, notochord; v, velum transversum. 



the result of division of a single outgrowth of the roof. The anterior 

 of these is the parietal organ or eye; the other the pinealis 

 or epiphysis proper. The two vary in development in different 

 vertebrates, the parietal eye being well-marked only in cyclostomes, 

 Amia, teleosts and most lizards, while the pinealis is almost invariably 

 present. 



In its fullest development in lizards (fig. 162) and Sphenodon the 

 parietal organ extends as a slender stalk, hollow at first, through the 

 parietal foramen of the skull, expanding beneath the skin to a vesicle, 

 abovcjwhich the integument is usually thin and transparent, forming a 

 physiological cornea. The distal wall of the vesicle is thickened in 



