1886.] 



On the Median Eye in Lrizards, 



561 



(1.) External Appearance. 



The organ is sitnated upon the dorsal surface in the median line, 

 and at varying distances posterior to the level of the paired eyes ; the 

 presence or absence of an external indication of the organ can be by 

 no means relied upon as indicating the existence or non-existence of the 

 structure in a highly developed state. In many cases, as Varanus Ben- 

 gaJensis, the various species of Calotes and Lacerta ocellata, the organ is 

 marked externally by the presence of a specially modified scale, usually 

 considerably larger than the surrounding ones, and with a circular 

 patch of pigment behind the whole resembling a cornea. Being- 

 transparent, and forming the anterior boundary of a capsule con- 

 taining the organ, the appearance of a dark pupil surrounded by a 

 light circle is produced. On the other hand, as in Plica umbra or 

 Cyclodus (sp.), a more or less highly specialised scale may be present, 

 but the organ beneath be not highly developed ; or again, as in 

 Hatteria, there may be no special scale, but only a general trans- 

 parency in the median line immediately above the organ, which may 

 nevertheless be in a highly developed state. 



(2.) Position of the Organ. 



The organ may lie at different levels imbedded more or less deeply 

 in connective tissue beneath the skin, or even within the skull cavity, 

 but is always placed external to the dura mater. It always has a 

 definite relationship to the parietal foramen usually lying within this. 

 In Calotes it is placed immediately beneath the specially modified scale ; 

 in most forms, such as Varanus, Seps, Anolius, Leiodera, &c, it lies 

 within the foramen, and separated by specially modified connective 

 tissue from the skin. In Hatteria it lies on the inner side of the 

 foramen, which is filled up by a plug of connective tissue, and in 

 Lacerta ocellata the bone around the foramen is modelled to fit closely 

 to the outline of the organ and the connective tissue surrounding 

 this. 



(3.) Structure of the Organ. 



It may be said at once that Leydig's "frontal organ " resembles in 

 essential structure an invertebrate eye. 



This resemblance has lately been clearly pointed out by Graaf, in the 

 case of Anguis fragilis, and is found to hold good for many others. 

 He in common with all previous observers regards the organ as the 



been allowed to examine duplicate specimens of ten species from the British 

 Museum ; they are not all described in this communication, but will be dealt with 

 more fully subsequently. By Professor C. Stewart's kindn. ss also I have been able 

 to examine duplicate specimens of Iguana and Varanus from the museum of the 

 Royal College of Surgeons. 



VOL. XL. 2 P 



