SENSE ORGANS. 



85 



quently becomes the twixt brain are best considered in 

 connection with the sense organs. These are best developed 

 in the lizards, and hence these animals serve as the basis of the 

 following account. At an early stage there arises from the 

 epithelial roof of this region a hollow outgrowth (the epiphysis) 

 directed upwards and forwards, its distal end at first being in 

 contact with the epidermis of the top of the head. The extrem- 





>'^' 



Fig. go. Section of pineal eye of Hattei'ia, after Spencer from Wiedersheim., 

 a, capsule ; b, lens ; 6, vesicle ; d, retina ; e, molecular layer ; f, blood-vessels ; 

 g\ large cells ; h^ nerve. 



ity of this outgrowth expands into a more or less spherical 

 vesicle, the pineal or parietal eye, which may come to lie at some 

 distance in front of its point of origin, above the cerebral hemi- 

 sphere, and usually on the right side. The cells of this pineal 

 vesicle increase in number, producing a thickening of the walls ; 

 while the distal and proximal surfaces of the vesicle become 



