I.ACERTILIA. 345 



be a simple epithelium, but more often they present peculiar 

 modifications which recall the structure of an eye. In such cases 

 the dorsal wall is thickened in a manner which suggests a lens- 

 like structure (Fig. 189) ; while the rest of the wall is pigmented, 

 contains several layers of nuclei and generally presents an aspect 

 which recalls that of a retina. The parietal organ is sometimes 

 quite separate from the brain (Calotes, Seps, etc., Fig. 190) but 

 more usually perhaps it is connected with the pineal body by 

 a cord of tissue. In Cydodus its cavity is continuous with that 

 of the pineal stalk and so with that of the 3rd ventricle. On 

 account of the eye-like structure and also because it is often 

 attached by a cord of tissue to the pineal body or to the roof of 

 the thalamencephalon just in front of and in close connection 

 with that body, the parietal organ is sometimes spoken of as the 

 " pineal eye." The hypothesis as to its nature suggested by this 

 name must be received with caution, as will be explained later on. 

 It is usually placed close to the skin of the top of the head and 

 the skin over it is frequently without pigment. In such cases a 

 portion of the scale immediately overlying it may have a cornea- 

 like appearance. There is however no relation between this 

 external indication and the degree of eye-like development of 

 the parietal organ ; e.g. in Hatteria in which it has a more eye- 

 like structure than in any other form, there is no external indi- 

 cation of it on the top of the head, while in Ceratophora, in which 

 the modified scale is present, there is no parietal organ. 



The eye-like character is not always discernible, e.g. in Cydodus in which 

 though it lies in the parietal foramen and has a modified scale -over it, it 

 appears to be nothing more than the distended end of the pineal stalk 

 (pineal body), in Chamaeleon and others there is no pigment and the walls 

 of the vesicle show no retinal or lens-like differentiations, though the 

 vesicle is connected to the pineal body (or stalk ?) by a solid cord. In others 

 again in which the eye-like features can be detected there is considerable 

 variety as to the extent to which they are differentiated, and also as to the 

 presence or absence of a connection (always solid when eye-like structure 

 is present) with the pineal body (Fig. 190). Lastly in forms in which the 

 parietal foramen is closed (e.g. Gecko, Ameiva, Ccratophora) the parietal 

 organ is absent, and the pineal body ends just within the skull- wall. 



Developmentally the parietal organ appears to ariso as a diverticulum 

 from the anterior side of the pineal body or directly from the cerebral 

 roof immediately in front of this organ (see p. 70). 



So far as can be ascertained from experiment the parietal organ has no 

 trace of a visual function, and the interpretation of it as the vestige of a 

 once functional median eye rests entirety upon the evidence of histology. 

 But the histological evidence is by no means conclusive. The nervous 



