256 THE PINEAL ORGAN 



the general statement with regard to the frequency of pigment cells in the 

 centre of the wide type of lens. 



Besides variations in the form of the lens in the parietal eye of reptiles, 

 cases occur in which the lens is absent, its place being simply marked by 

 a thin part of the wall of the vesicle which is destitute of pigment, and 

 although not an homologous structure, it resembles the cornea of the 

 lateral eyes ; a good example of absence of the lens was described by 

 Spencer in a specimen of Moloch horridus. This was, however, a very 

 exceptional case in which the parietal eye was almost completely filled with 

 pigmented tissue, leaving only a small space in front, like the anterior 

 chamber of a lateral eye. The parietal organ lay in the parietal foramen, 

 and was connected by a solid stalk with the distal end of the epiphysis. 

 The specimen appears to be a case of arrested development in which the 

 parietal organ has failed to separate from the parent stem, and has failed 

 to differentiate into lens and typical retina. If this interpretation is 

 correct the end vesicle of the epiphysis has also failed to develop. It is 

 possible, however, that the structure described by Spencer as the parietal 

 organ, since it was directly connected by a stalk with the epiphysis, was 

 itself the end vesicle of the pineal organ and it was the parietal organ which 

 was lacking — a view which was first suggested by Studnicka. 



The frequent appearance of pigment in the centre of the lens of the 

 parietal organ, and the occasional presence in such cases of a notch on the 

 under surface of the lens or of a seam on the superficial surface, may be of 

 significance with reference to the theory of a dual origin of the parietal 

 organ. A parallel condition is met with in cases of cyclopia, whether this 

 anomaly occurs in single individuals or in double monsters, more parti- 

 cularly in certain cases of Cephalothoracopagus monosymmetros or 

 Diprosopus triophthalmos. In these cases varying degrees of blending 

 of the two eyes occur in the development of the cyclops eye, namely, 

 between simple approximation of two eyes lying in a single orbital cavity 

 and an apparently single eye in which a compound lens is found which is 

 formed by the fusion of two halves. Thus, according to the classification 

 of the various degrees in the development of cyclopia by Bock, with 

 reference to the fusion of the paired lateral eyes, cases occur in which : 



1. The two bulbs are united only by the sclera. 



2. The optic nerves are approximated ; the common scleral tissue 

 between the bulbs has become thinner ; the cornea, iris, lens, vitreous, 

 and retina are double. 



3. The cornea is single ; the iris, lens, vitreous, and retina double, the 

 optic nerves separated merely by a narrow space filled with connective tissue. 



4. One cornea ; two lenses united in the median plane of the compound 

 eye ; sclera, chorioidea, retina, and optic nerve single. 



