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 

 Reptiles, in 

 H a 1 1 e.r i a > 

 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 

 Invertebra t e s 

 (fig. 245). The 

 portion of its 

 wall which lies 

 next to the sur- 

 face of the body 

 has been traiis- 



Fig. 245. Longitudinal vertical section through the pineal eye of 



Hatteria punctate and its connective-tissue capsule, after BALDWIN 



SPENCER. Slightly enlarged. 



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

 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. 



8t 



formed into a 

 lens-like struc- 

 ture (F) ; 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 surfac3 of which 



