THE PINEAL ORGAN OF MAMMALS 



311 



formed by a median condensation of the loose mesenchyme which occupies 

 the space between the developing hemispheres. It is not formed by the 



D.D! 



D.D'.' 



O.R 



Fig. 



Aq.C. 



216. — Median Linear Reconstruction of the Pineal Region of a 

 4 1 -month Human Fcetus showing Anterior Opening of Aqueductus 

 Cerebri ; the Columnar Epithelium of the Subcommissural Organ 

 between this and the Posterior Commissure, above which is the Pineal 

 Organ, consisting of a Main Posterior Lobe which in the greater part 

 of its extent is solid and a Small Anterior Lobe. Above and in front 

 of the Pineal Recess is the Superior Commissure and above this the 

 Bifid Dorsal Diverticulum. (R. J. G.) 



A.L. : anterior lobe. 



Aq. C. : aqueductus cerebri. 



C. Ep. : columnar epithelium. 



D.D.', D.D." : dorsal diverticulum. 



P.C. : posterior commissure. 



P.L. : posterior lobe. 



P.R. : pineal recess. 



Q.P. : quadrigeminal plate. 



S.C. : superior commissure. 



S.C.O. : subcommissural organ. 



union of the two layers of a fold, and the same remark applies to the forma- 

 tion of the tentorium and the falx cerebelli. 



The recess (Fig. 213, B, Post. D.) which projects backward from the 

 posterior wall of the main diverticulum probably corresponds to the 

 posterior pineal diverticulum described in the preceding specimen 



