Sorensen, Study of Epipliysis and Roof of Dienccplialon. 165 



terest among investigators as the parietal eye. Since Leydig's 

 first study and discovery of the organ (p. 23), and De Graaf s 

 important histological work (p. 28) giving to the organ a visual 

 function, much time and attention has been given to its study 

 by able authors. Among those who have written upon the 

 parietal organ are: Spencer on Lacertilia (p. 31), McKay on 

 Hinulia (p. 33), Beard on Ammoccetes and Petromyzon (p. 31), 

 Klinkowstrom on Iguana (p. 58), Beraneck on Lacerta and An- 

 guis (p. 60), and Studnicka on Ammoccetes and Petromzon 

 (p. 50). Among the various functions assigned to the organ 

 we might mention that by Rabl-Riickhard who thought it might 

 serve as an organ of temperature sense, and that of Susanna 

 P. Gage, who suggests that it might serve as a nutritive organ 

 in the ontogenetic development. Most investigators, however, 

 are now agreed that it is a degenerate visual organ. The pari- 

 etal organ which we are about to describe has every appearance 

 of a degenerated eye. It is situated in the meson, above the 

 anterior half of the prosencephalon, within the foramen on the 

 top of the skull, and immediately beneath the subcutaneous 

 layer. The entire organ is surrounded by a delicate connective 

 tissue capsule. In the figure (Plate XVII) the capsule is seen 

 to be in connection with the surrounding tissues by connective 

 tissue or possibly muscular strands. Within the capsule are 

 seen the lens and retinal layers clearly differentiated. The lens 

 is composed of columnar cells with darkly stained nuclei which 

 are situated in the inner one half of the cells. The cells are for 

 the most part parallel. The retina is seen to be composed of a 

 double layer of densely pigmented cells. The sections were 

 stained with haematoxylin and only the nuclei were visible. 

 The inner layer of nuclei were more densely 'pigmented than 

 the outer but were clearly made out with the twelfth inch and 

 drawn with the camera. Close and careful study of the two 

 layers revealed the fact that they are not two distinct layers, but 

 are continuous. This would lead us to suggest that possibly 

 we have here a development analogous to that found in the 

 paired eyes, i. e. that the lens is formed from the skin and the 

 r etina, growing out from the brain as the epiphysis, is con- 



