130 i I;KDERICK TILNK.V AND LI TIIKK r. WAKKKX 



ganglion niul passed close behind the first nerve to the parietal 

 eye. The lens of the parietal eye is not uniform in its shape; 

 it occurs in the following different forms: 



1. Regular bi-concave lens, both surfaces curved, which is 

 most common in Lacc/in r/npara, Laccrln ugHi* and Lnariu 

 L< iulm max ///tiilus, Scps cJiiilciilica. Phrynosoma dng- 

 and Sphenodon. 



'2. Bi-convex, with the under surface more convexed than the 

 upper, as in Anolfs and Sphenodon. 



3. Plano-convex, as in Anyni* and Iginunt. 



1. ( 'oncavo-convex, as in Calotes, Vannm* l><//</altns/x, and 

 }', i run us (lit/tuilcHS. 



'I'lie structure of the lens is ma.de up of peculiar, long, cylin- 

 drical cells apparently derived from modified ependymal cells. 

 These are the so-called lens cells. There are some intercellular 

 spaces, probably lymph spaces, according to Leydig ('91). 23 

 The lens cells are nearly free of pigment. The substance of these 

 cells is very hard. Their nuclei are oval or round and are sel- 

 dom scattered over the entire lens surface or its entire thick- 

 ness. They are most numerous at the border of the lens where 

 the latter passes over into the retina. 



The parietal foramen. Ley dig" in 1872 found a round or oval 

 opening in the skull of N/,7/."//m/r>// situated in the osparietal, 

 which seemed eil her directly to serve as the outlet for the parietal 

 organ or else for the entrance of light rays. It was reminiscent 

 of a similar opening in the cartilaginous roof of the cranium in 

 selachians. In most cases the parietal eye is in, or directly 

 under, this foramen. Species which do not possess a parietal 

 eye have a parietal foramen which is filled by the pineal organ, 

 in which ca.se, the end-vesicle takes the place of a third eye as 

 far as location is concerned. The foramen is absent in a large 

 number of saurians, particularly in the ( ieckonidae, and it is 

 also absent in ('crato/ilmni UX/H r<i. There are also instances in 

 which the foramen does actually appear in some individuals of a 

 species and yet in other individuals of the same -.pecies it is closed 

 by bone. The eye usually lies in the middle of this foramen or 

 near its upper edge. The relation be! \\een eye and foramen is 



