284 THE PINEAL ORGAN 



Owsjannikow's specimen differed from Spencer's in the more complete 

 differentiation of the parietal eye, a definite lens and retina being dis- 

 tinguishable. He described two layers in the retina : an inner composed 

 of deeply pigmented, long, rod-like cells bearing cilia, and an outer 

 layer which was devoid of pigment and contained fibres and cells with 

 round nuclei. The lens was biconvex, contained no pigment and was 

 formed of elongated cells. In the cavity of the vesicle there was a fine 

 granular mass, which represented a vestige of the vitreous body. The 

 conclusion to be drawn from these two widely different examples is that 

 a variable degree of differentiation of the parietal organ may occur in 

 different specimens, the development proceeding farther in some cases 

 than in others, in which it becomes arrested at an early stage — the arrest 

 being followed by degenerative changes. Another interpretation of the 

 differences between these two specimens is the possibility of the hollow 

 structure devoid of pigment, which was described by Spencer as the 

 parietal eye, being the end vesicle of the pineal organ, the parietal eye 

 having failed to develop or having been present only for a short period 

 during embryonic development, as in the examples of disappearance of 

 the organ described by Legge in Gongylus (p. 281). 



The Pineal System of Ophidia. 



The pineal system of the snakes is represented only by the epiphysis, 

 the parietal eye being absent. In one case, however, published by Hanitsch 

 in an embryo of the common viper (Pelias berus) a well-differentiated 

 parietal organ with a lens, which contained much pigment in its interior, 

 was described. The epiphysis had the usual form, and had the appear- 

 ance of twisted tubes between which connective tissue was interposed, 

 giving it a glandular appearance. The case seems, however, to have been 

 exceptional and the parietal organ has not been seen in other species. In 

 view of the temporary embryonic existence of a parietal organ in Gongylus 

 (described by Legge, p. 281), the correctness of this single observation 

 cannot be denied, and the very occasional occurrence of a parietal eye in 

 snakes must be regarded as awaiting further confirmation. 



The skull in living species of snake has no parietal foramen, and in 

 the python the parietal bones are fused in the median plane so as to form 

 a prominent keel-like ridge in which it is unlikely that any remnant of 

 a parietal canal would persist. A foramen is, however, present in the 

 skull of the extinct Pythonomorpha, in which the parietal bones, although 

 united, are not produced upwards as an intermuscular ridge, but form a 

 flat, triangular plate with the parietal foramen near or at its anterior 

 border. Examples are met with in Clidastes propython (Cope), Upper 

 Cretaceous, Alabama ; and Platecarpus coryphceus (Cope), Upper Cre- 



