234 THE PINEAL ORGAN 



is shown in Fig. 166 — A, in sagittal section ; and B, in transverse section. 

 It commences on the dorsal aspect of the epiphysis near its apex and 

 courses backwards either on or in the substance of its dorsal wall, and 

 finally enters the ventral part of the posterior commissure. According to 

 Braem the fibres composing the tract arise from ganglion cells in the 

 posterior commissure, while Haller states that the tract arises by two roots 

 from the part of the thalamus which lies medio-ventral to the posterior 

 commissure. 



Gaupp 1 also describes a bundle of nerve-fibres which he saw in one 

 specimen on the ventral side of the stalk of the epiphysis. 



The Epiphysis or Proximal Part of the Pineal Organ of Anura 



The epiphysis (Fig. 166, p. 232) usually has the form of a hollow tube 

 ending blindly in front and opening posteriorly in the third ventricle, 

 just behind the habenular commissure. Occasionally in the adult animal 

 the lumen has disappeared in its basal part, which thus forms a solid stalk. 

 In transverse section the tube is seen to be oval in outline, as if dorso- 

 ventrally compressed, the lower wall being thicker than the upper. 

 On each side a row of short lateral diverticula are present, which according 

 to Braem in Rana are eight in number, the more anterior diverticula 

 being larger than the posterior. The epiphysis thus has a glandular 

 appearance like that in certain teleostean fishes, e.g. Salmo purpuratus. 

 This condition we shall see occurs also in reptiles, birds, and mammals, 

 but in these attains a much higher degree of differentiation. 



The Character of the Epithelial Cells forming the Walls of the Epiphysis. — 

 Those next to the lumen tend to become columnar in type, and some- 

 times bear cilia or cylindrical protoplasmic processes which project into 

 the lumen, as in the stalk of certain teleostean fishes. These have been 

 regarded by Galeotti as secretion processes, but Studnicka states that 

 their rod-like form in the adult animal is more like that of sensory- 

 epithelial cells, such as are seen in the parietal organ of Petromyzon. 

 These cells are especially abundant in the lower wall of the epiphysis, 

 where there is an outer layer of rounded cells in the situation of the 

 ganglion cells of the parietal organ of the lamprey. The cells in the 

 dorsal wall of the epiphysis are, like those of the ventral wall, ependymal 

 in character, but are mostly round. In its distal part the pineal tract is 

 found imbedded in the wall of the tube, but near the base it becomes 

 superficial and lies in a groove on its dorsal aspect. The general im- 

 pression which is given by examination of the structure of the epiphysis 

 is that of an aborted neuro-epithelial tissue, the attempted differentiation 

 of which has become arrested. 



1 E. Gaupp. Zirbel, Parietalorgan und Paraphysis. Ergebnisse der Anat. und Entwick- 

 hmgsgeschichte, von Merkel und Bonnet. 



