434 



EMBRYOLOGY. 



(st), which consists of fibres and spindle-shaped cells, and bears a 

 certain resemblance to the embryonic optic nerve ; and thirdly into 

 a hollow, funnel-shaped projection (A) of the roof of the between- 

 brain, which still exhibits here and there sac-like enlargements. 



In a third 

 division of the 

 Hep tiles, in 

 Hatteria, 

 Monitor, the 

 Blind-w o r m s, 

 and Lizards, 

 the vesicular 

 distal portion 

 of the pineal 

 gland under- 

 goes a striking 

 metamo r p h o- 

 sis, by means 

 of which it ac- 

 quires a certain 

 resemblance to 

 the eye of many 

 In vertebra t e s 

 (fig. 245). The 

 portion of its 

 wall which lies 

 next to the sur- 

 face of the body 

 has been trans- 

 formed into a 

 lens-like struc- 

 ture (I) ; the 

 part of the wall 

 lying opposite 

 the latter and 

 continuous 



with the fibrous cord (St) has, on the contrary, been converted into 

 a retina-like structure (r). The formation of the lens (I) is due to 

 the fact that the epithelial cells of the anterior wall of the vesicle 

 have become elongated into cylindrical cells and uninucleate fibres, 

 and have thereby produced an elevation, the convex surface of which 



ft 



Fig. 246. Longitudinal vertical section through the pineal eye of 

 Hatteria punotata and its connective-tissue capsule, after BALDWIN 

 SPENCER. Slightly enlarged. 



The anterior part of the capsule fills up the parietal foramen. 



K, Connective-tissue capsule ; I, lens ; h, cavity of the eye filled with 

 fluid ; r, retina-like portion of the optic vesicle ; M, molecular 

 layer of the retina ; g, blood-vessels ; x, cells in the stalk of the 

 pineal eye ; St, stalk of the pineal eye, comparable with the optic 

 nerve. 



