THE PINEAL SYSTEM OF REPTILES 



245 



the anterior diverticulum is at first situated to the left of the posterior 

 part, and the nerve which arises from the retina of the parietal eye, at a 

 later stage of development is connected with the left habenular ganglion — 

 Dendy, Sphenodon, 1899 ; Gaskell, 1908 ; Patten, 1890 ; and others. 



Dendy's view of the bilateral origin of the parietal sense organ, which 

 was also held by Hill, Gaskell and others, is supported by the geological 

 evidence of the existence in certain extinct fishes of two parietal impressions 

 or foramina, placed side by side in the roof of the skull, as seen in Figs. 

 140, 229, 230 (Chap. 18, p. 200, and Chap. 23, pp. 329, 330), of 

 Dipnorhynchus, Pholidosteus, Rhinosteus, and Titanichthys. It also receives 

 strong support from the occasional development of an accessory parietal 



Fig. 175.— Sagittal Section through the Parietal Foramen of a Horned 

 Lizard (Phrynosoma coronatum). 



Lying in the foramen are a parietal eye and an accessory parietal organ. (After 

 Ritter, from Die Parietal-organe Studnicka.) 



P.E : parietal eye. A. P.O. : accessory parietal organ. 



sense organ which lies close to or is in continuity with the tip of the 

 pineal organ, as in Pseudopus palassii, described by Studnicka in 1905 

 (Figs. 174, 177), and Phrynosoma coronatum (Fig. 175) (after Ritter). 



The nerves and nerve Ganglia of the Pineal System of Reptilia. 



A detailed study of the nerves of (1) the parietal sense organ and 

 (2) the pineal organ, including the pineal sac or end vesicle with the stalk 

 and the basal or proximal part, is instructive from several standpoints, more 

 particularly (A) as an aid to the solution of the question as to its origin — 

 namely, whether from two serially homologous parts placed one behind the 

 other or from vestiges of primarily bilateral sense organs ; (B) ascertaining 

 whether the pineal system shows signs, both phylogenetically and onto- 

 genetically, of degeneration, or of the evolution of an endocrine organ. 



An important point to bear in mind is that no nerve-fibres are found 

 in connection with the rudiment of the parietal sense organ until after 

 the vesicle has been completely separated from the basal part of the 



