1886.] 



Ofi the Median Eye in Lizards. 



559 



variety of graphite is characterised by giving a very large yield of 

 marsh-gas. 



C0 8 45-42 



CO" 39-88 



CH 4 4-43 



H 8-31 



N 2-00 



Occluded gases in volumes of the graphite=53 " 13. 



XIV. " Preliminary Communication on the Structure and 

 Presence in Sphenodon and other Lizards of the Median 

 Eye, described by von Graaf in Anguis fragilis" By W. 

 Baldwin Spencer, B.A., Demonstrator of Comparative 

 Anatomy in University of Oxford, Fellow of Lincoln College. 

 Communicated by Prof. H. N. Moseley, F.R.S. Received 

 June 10, 1886. 



In 1872 Leydig* described a structure iu Lacerta agilis, L. muralis, 

 L. vivipara, and Anguis fragilis, to which he gave the name of "frontal 

 organ." 



In the embryo, owing to its being deeply pigmented, it forms a 

 prominent feature on the roof of the original forebrain in connexion 

 with the pineal gland ; in the adult it lies immediately beneath the 

 skin, and, according to him and subsequent observers, completely sepa- 

 rated from the brain. 



In Anguis fragilis the organ is seen microscopically to consist of long 

 cells like those of a cylindrical epithelium, which are so arranged that 

 together they form a shallow pit with a circular outline. The edge of 

 the pit is directed downwards, and has a thick black girdle of pigment. 

 It corresponds in position to that occupied by the parietal foramen 

 in the adult. 



Leydig regarded our knowledge of the organ as insufficient to allow 

 of any statement being made with regard to its function. 



Rabl-Ruckhard,t in 1882, describing the development of the pineal 

 gland in the trout, pointed out the resemblance between its develop- 

 ment as a hollow outgrowth of the brain and that of the optic 

 vesicles. 



Granted such secondary developments from the epiblast and meso- 

 blast as combine to produce the eye, and which are absent in the case 

 of the pineal gland, though the distal extremit}" of the latter lies in a 



* " Die in Deutschland lebenden Arten der Saurier," p. 72, taf. 12. 

 f " Zur Deutung und Entwickelung des G-eliirns der Kuoclienfisclie." Arch, 

 fur Anat. u. Pliys., Jalirg. 1882, p. 111. 



