THE PINEAL ORGAN OF MAMMALS 313 



section of the wall of the pineal sac of an adult Sphenodon. 1 In the 

 illustration, Fig. 217, it will be observed that the wall of the pineal sac is 

 folded, and that between the hollow epithelial outward projections of the 

 wall there are ingrowths of vascular connective tissue. Such an ingrowth 

 is seen in the centre of Fig. 217, D, and it will be noted that the vascular 

 core, B. V., which lies in the middle of the lobule is surrounded on all 

 sides by neuroepithelium. Should a cross-section of such an apparent 

 lobule be examined, it will be seen to consist of a core of sinusoidal blood- 

 vessels surrounded by a perivascular sheath of pial tissue, which is in 

 contact peripherally with the external limiting membrane of outgrowing 

 neuroepithelial processes. Proceeding farther outward from the central 

 pale area formed by the vessels and their loose mesenchymatous sheath, 

 there will be found beyond the limiting membrane the reticular and 

 nuclear zones, the latter containing pale cells with large vesicular nuclei. 

 Finally, there is a peripheral zone of deeply stained ependymal cells and 

 the internal limiting membrane. 



The deeply stained cells surrounding the clear vascular areas have 

 the appearance of epithelial cords cut in various directions. When the 

 section is transverse (Fig. 217, F) the cords appear as rings of cells, 

 " rosettes," with deeply stained oval nuclei arranged radially round a 

 small palely stained central zone, which is formed by the inner ends of 

 the cells coming into contact in the central axis of the cord. If the section 

 of the cord is longitudinal, two parallel rows of nuclei are seen, which 

 are separated by a palely stained axial zone, where the inner, palely 

 stained ends of the cells come into contact in the situation of a virtual 

 lumen. These cords of proliferating ependymal cells are usually grouped 

 in lobules (Fig. 214, D), which grow outward between the vascular 

 ingrowths of the pia mater. 



In the later stages of development neither the internal nor external 

 limiting membrane is visible, and it appears that the lumen of the out- 

 growing epithelial lobules, seen in Fig. 217, A and B, is replaced by a 

 virtual lumen, which forms the central axis of the solid cords which are 

 seen in cross-section in Fig. 214, D, and Fig. 217, F. The external 

 limiting membrane also disappears, as it has been shown to do in birds, 

 and the ependymal tissue mingles with the connective tissue, as was 

 clearly demonstrated by Studnicka to be the case in Strix (Fig. 204, 

 p. 297). 



1 This specimen is from one of a series of microscopical sections illustrating 

 the development and the structure of the adult pineal region of Sphenodon and 

 Geotria which were prepared by the late Professor Dendy of the University of 

 London, King's College, and we take this opportunity of thanking Professor 

 D. Mackinnon for permission to make use of this most valuable collection in 

 our recent investigation. 



