1886.] 



On the Median Eye in Lizards, 



563 



processes from the structures on both sides of the layer seem to run 

 into its substance. (4.) External to the molecular layer an outer part, 

 in which three kinds of elements may be distinguished : (a) round 

 nucleated cells, somewhat larger than those of the inner layer ; (6) 

 rod-like structures, somewhat conically shaped, with their broad ends 

 external ; (c) small nucleated spindle-shaped elements, placed between 

 the latter at their bases. (5.) Though difficult to trace, a fine layer 

 of nerve fibres appear to spread round the vesicle from the nerve 

 which enters it at the surface nearest the epiphysis. The elements 

 are connected serially, though processes from the rods may be seen 

 passing at times directly into the molecular layer, or the rod elements 

 externally, or even right through to the external surface. Such pro- 

 cesses are accompanied by pigment, and may in some cases merely 

 indicate supporting structures. 



In Hatteria, as in other forms, a special bundle of rods lying in the 

 optic axis is highly developed, being much lengthened out and 

 running down into the nerve, their outer extremities being in connexion 

 with a particular group of nucleated cells. 



Von Grraaf describes in Anguis a layer of small rod-like structures, 

 similar apparently to those found in many invertebrate eyes, though he 

 is not certain as to their nature. In Hatteria and other forms examined, 

 the vesicle appears to have been filled during life by a fluid material, 

 and this in coagulating adheres to the wall. The coagulation often 

 apparently sets in from definite points, and these being the ends of 

 rods, gives the appearance, under certain conditions of light, of 

 refracted processes attached to these structures. 



The nerve enters the vesicle posteriorly, certain fibres appear to enter 

 into connexion with the cells connected with the specialised rods, the 

 remainder spread out around the external surface of the vesicle, and 

 here enter into connexion with the elements, that is, the rods 

 bound the internal surface of the vesicle, and the nerve-fibres the 

 external. 



The nerve, whilst differing in appearance from an ordinary one, 

 yet resembles more closely than anything else the developing optic 

 nerve, being formed of long spindle-shaped elements, which recall the 

 stage passed through when the at first round cellular elements of the 

 optic stalk are gradually lengthening out. The nerve in both cases 

 developes in a similar manner. 



The whole eye lies in a special capsule of connective tissue into which 

 enters and breaks up a blood-vessel, this vessel being present in con- 

 nexion with the eye in all Lacertilia examined, even in those in which 

 a nerve could not be distinctly traced. 



(b.) Lacerta ocellata. — In this form the organ lies considerably below 

 the surface, and so shut in by bone that it may be said to lie within 

 the skull. The dura mater which surrounds it is deeply pigmented, 



2 r 2 



