248 THE PINEAL ORGAN 



existence in a rudimentary form and the presence in it of pigment in 

 some species such as Pseudopus Palasii (Studnicka) (Figs. 174, 177) and 

 the occasional occurrence of a fibrous connection between the tip of 

 the pineal organ and the parietal eye such as occurs in the early develop- 

 mental stages of certain Amphibia, e.g. Rana temporaria, Braem (Fig. 161, 

 Chap. 19, p. 228), is of the greatest significance. It affords additional 

 support to the hypothesis based on the ontogenetic development of the 

 pineal organ and on geological evidence that the proximal part and stalk 

 of the end vesicle represent the common stalk of a primarily bilateral sense 



ac' A par.o. 



Fig. 177. — Sagittal Section through the Parietal Region showing a Large 

 Parietal Foramen, containing a Parietal Organ, the End Vesicle of 

 the Pineal Organ and Two Accessory Organs, from an Adult Example 

 of Pseudopus Pallasii. (After Studnicka.) 

 ac.', ac." : accessory organs. Hem. : hemisphere. 



bl. : blood vessel. 



c. hab. : habenular commissure. 



d.s. : dorsal sac. 



cp. : epiphysis. 



MB. : midbrain. 



par. 0. : parietal organ. 



pin. : end vesicle of pineal organ. 



st. : primary stalk of pineal organ. 



organ, the end vesicles of which in some types are completely absent ; 

 in others, one of the sense organs has become more highly evolved, and 

 retaining its connection with the superficial structures becomes separated 

 from the parent stem ; while the other, the growth of which early becomes 

 arrested, either retains its original connection with the common stalk or, 

 like the parietal sense organ, becomes separated off as an accessory 

 pineal organ. Such an accessory organ may lie close to the parietal sense 

 organ ; between this and the pineal organ (Fig. 177) ; or in continuity 

 with the tip of the pineal organ. The frequent presence of pigment in 



