286 



THE PINEAL ORGAN 



stages the epiphysis had the form of a simple tubular evagination of the 

 roof of the thalamencephalon, which in later stages, becoming thinner 

 at its basal or proximal part, developed a secondary stalk. Still later, by 

 a further attenuation of the stalk, the epiphysis became completely 

 separated from the roof of the brain. A detailed description of the 

 parietal region in an embryo of Chelydra (Fig. 195) shows that this region 

 of the roof of the brain differs little from that of other reptiles. The 

 tubular epiphysis springs from the roof of the third ventricle in the 

 usual situation in front of the posterior commissure and passes upwards 

 on the posterior wall of the dorsal sac, and then forwards over its superficial 

 surface as far as the velum and tip of the paraphysis. Its lumen widens 



MB. 



Fig. 195. — Parietal Region of the Brain of an Embryo of a Snapping Turtle 

 (Chelydra serpentina), showing the Relation of the Epiphysis to the 

 Dorsal Sac and Paraphysis. (After Humphrey, 1894.) 

 Cp. : posterior commissure. MB. : midbrain. 



Ds. : dorsal sac. Par. : paraphysis. 



Ep. : epiphysis. PL Ch. : plexus choroideus. 



V. : velum. 



as it passes forwards and the wall of the epiphysis, which in the basal 

 part consists of a single layer of epithelial cells, shows two layers of nuclei 

 at its rounded distal end. 



In the adult animal the lumen in some cases is found to have dis- 

 appeared, in the stalk and also in the body of the epiphysis ; in other 

 cases it may persist, and in one specimen of Cistudo europcea described 

 by Faivre it contained small granules of calcium phosphate. The shape 

 of the organ varies, being described as ovoid, conical, or tuberous ; it 

 is surrounded by blood-vessels and ingrowths of fibrous tissue may 

 penetrate the wall, giving a lobulated appearance to its surface, but there 

 is no outgrowth of epithelial buds or true lobulation. The structure 

 of the epiphysial wall of an adult specimen of Cistudo europcea was 

 described by Studnicka (1905) ; the wall was thick and penetrated here 



