THE BRAIN 171 



The olfactory lobes may be well marked or entirely invisible 

 externally. In such forms as Anguis, Amphisbsena and Typhlops 

 they are closely applied to the hemispheres, while in others (e.g., 

 Hatteria, Lacerta, Crocodilus) each consists of a well-marked olfac- 

 tory tract, passing anteriorly into an olfactory bulb from which the 

 nerves of smell arise. Olfactory ventricles are usually present. 



The thalamencephalon is always depressed, and is hardly, or 

 not at all, visible from the dorsal side. A distinct hypophysis and 



L ':> . ^ sj 



FIG. 141. LONGITUDINAL SECTION THROUGH THE PARIETAL EYE AND ITS CON- 

 NECTIVE-TISSUE CAPSULE OF Hatteria punctata. (After Baldwin Spencer. ) 



cp, connective-tissue capsule ; r, "lens ;" cr, cavity of the eye, filled with fluid ; 

 r l , retinal portion of the vesicle ; vs, blood-vessels ; C.M, cells in the nerve 

 stalk (s.n.). 



infundibulum as well as an epiphysis are present, and in Lizards 

 the parietal organ retains more or less distinctly, even in the adult, 

 its primitive structure as a median eye. 



This parietal eye (Fig. 141) is situated in the parietal foramen 

 of the skull, and is in close connection with the epiphysis, though 

 in the embryo the nerve which supplies it is seen to arise in- 

 dependently from the brain, in front of the pineal outgrowth. The 

 eye has the form of a vesicle, the dorsal wall of which may become 

 thickened to form a transparent lens-like body, while the rest of 



