THE PINEAL ORGAN OF MAMMALS 



305 



tliick ependymal layer (Ep.Z.), a thin middle layer or nuclear zone (N.Z.), 

 and an outer reticular or marginal zone (M.Z.). The inner ends of the 

 ependymal cells which immediately surround the lumen are clear and 

 destitute of nuclei ; they show a radial arrangement and are bounded 

 by an internal limiting membrane (Figs. 209, 210). The outer zone is 

 limited by a less defined external membrane and pia mater, and it is in 

 relation with large endothelium-lined spaces and capillary vessels which 



P.Co. V - S - 



KM. 





Y 



3 



\i< ••'."•.•■."•••.'• ■■.•••••.•'•:.•-••-• ^.^%a^t?r'^v 



JH8A\. v -L« - 



;N.\ 



■-- : ::'vi-;-:;^^::-; 



77z. 



P.K 



CM. 



Fig. 211. — Paramedian Section through the Mid-brain of a Rabbit Embryo 

 (18 days), showing the direction of the fibres of the posterior com- 

 missure and their connections with the pons varolii and interpedun- 

 CULAR Region. (R. J. G.) 



CM. : corpus mammillaris. V.M. : ventriculus mesencephali. 



P. Co. : posterior commissure. V.S. : venous sinuses. 



P.V. : pons Varolii V.L. : ventriculus lateralis. 

 77? . : thalamus. 



lie in the surrounding loose mesenchymal tissue. The extent and thick- 

 ness of the posterior commissure at this stage of development are well 

 seen in Figs. 207, 208, which represent the region in a human embryo 

 and in a rabbit embryo at a corresponding stage of development (16 days, 

 11 mm.). Fig. 211, of a slightly older rabbit embryo (18 days), shows 

 the direction of the fibres of the posterior commissure as seen in a para- 

 median sagittal section. 

 20 



