PINEAL EYE IN LACERTILIA. 191 



bifnrcatus by a tubercle slightly depressed below the level 

 of the surrounding ones, and having a very transparent appear- 

 ance;^ it lies in the median line just in front of the anterior 

 end of the strongly marked ridge, which occupies the dorsal 

 surface of the head posteriorly. 



Fig. 40 gives a diagrammatic view of the relationship of the 

 different parts ; the parietal foramen is not large but is still 

 clearly present, and very easily distinguishable in sections. 

 Within it and lying immediately beneath the modified tubercle 

 is the optic vesicle ; elsewhere as usual the skin is deeply pig- 

 mented, but the pigment cells are entirely wanting above the 

 vesicle, a fact which is especially noticeable in sections of this 

 animal, the cells having long processes and being closely 

 packed together (fig. 21). It is this absence of pigment which 

 produces the transparent effect in the tubercle. The surface 

 of the latter is very convex, and beneath it the layers of the 

 skin are arranged as usual, a series of special connective-tissue 

 fibres forming an encasement for the vesicle. Within the 

 foramen there is the customary well-marked and branching 

 blood-vessel [b.v.), which accompanies the pineal stalk. 



Structure of Vesicle. — In Chameleo the structure of 

 the vesicle is very sirnple. It has the form of a hollow sphere 

 whose walls have been compressed dorso-ventrally, so that its 

 greatest length lies in the line of the long axis of the head. 

 Its walls are formed of elongated distinctly nucleated cells, 

 those facing into the cavity bearing long cilia; no pigment is 

 present and there is no differentiation into lens and retina, the 

 cells of the anterior and posterior walls of the vesicle being 

 alike. Posteriorly the inner wall of the vesicle is, as it were, 

 drawn downwards (fig. 21), a small horn-like space being thus 

 formed, turned somewhat towards the pineal stalk ; its general 

 appearance conveys the idea of the vesicle having at first had 

 the relationship to the then open pineal stalk which is at 

 present shown by the swollen distal extremity to the epiphysial 

 tube in Cy clod us. By the meeting of the walls of the epi- 

 physial tube the vesicle would become closed, and the solid 



^ The external indication is much clearer in some than in other specimens. 



