250 THE PINEAL ORGAN 



ganglionic nerve cells such as are present in the retina of the parietal sense 

 organ. The pineal sac is invested by an outer fibrous sheath, continuous 

 with the pia mater and containing blood-vessels. Beneath this is a 

 basement membrane or membrana limitans externa, while internally 

 next to the lumen is a less defined membrane, pierced by the inner 

 ends of cells forming the internal epithelial layer. Between these two 

 membranes is a supporting basis of ependymal glial tissue, the cell- 

 elements and fibres of which are arranged for the most part in a radial 

 manner. In addition to the supporting glial element, nerve-fibres are 

 present which form a definite layer running parallel to the surface and 

 about midway between the two limiting membranes. Some of these 

 fibres appear to be continuous with the tapering deep-ends of the sensory 

 cells of the inner epithelial layer, and others with processes of cells, 

 which resemble nerve-cells, belonging to the outer epithelial layer. In 

 young specimens of Sphenodon the wall of the pineal sac is less differentiated 

 and its structure resembles that of the stalk and proximal part. In all 

 three parts of the organ, the wall consists of a single or double layer of 

 ependymal cells, the oval nuclei of which are arranged radially to the 

 axis of the tube, and there is a tendency where the tube is narrow, as 

 in the stalk for the cells to form a single layer and become cubical in 

 form, whereas in the expanded segments of the tube, two or more layers 

 of nuclei are present and the cells become elongated. In adult specimens 

 of some species of reptiles, e.g. Sphenodon (Spencer), Anguis (Ley dig), 

 and Pseudopus (Studnicka) in which the proximal or " epiphyseal segment '* 

 becomes expanded, the walls of this part become thickened and in- 

 folded ; the spaces between the folds being filled with vascular con- 

 nective tissue, so that sections of it have a glandular appearance, which 

 closely resembles that of the paraphysis and of the choroid plexuses. 

 In older specimens there is a tendency towards degeneration in all 

 segments of the pineal organ. This is also attended by folding of the 

 walls and ingrowth of vessels. The degenerative processes are still 

 more marked in the epiphysis of Ophidia and Chelonia, there being an 

 early arrest of development and lack of differentiation which sometimes 

 results in the disappearance of the cavity of the epiphysis or that of 

 its constricted " neck " called by Studnicka the secondary stalk. In 

 late stages the vessels may disappear and intracellular and intercellular 

 spaces appear in a solid mass of degenerated glial tissue, as in Tropinodotus 

 (Fig. 179) (Leydig). 



There is the same difference of opinion with regard to the nature 

 of the processes projecting through the internal limiting membrane of 

 the distal part of the epiphysis that we mentioned in the case of teleostean 

 fishes (Figs. 148, 154, Chap. 18, pp. 211, 219), namely, as to whether these 



