1910,] On a Mesoccelic Recess in the Human Brain, etc. 521 



organ. The epithelium is seen to be losing its characteristic columnar 

 appearance, especially in the middle line, but Keissner's fibre is still present, 

 and, indeed, in this and neighbouring sections may be seen breaking up into 

 branches, which actually run to the epithelium. 



Fig. 5. — Cat. Transverse Section through the Region of the Posterior Commissure 

 considerably anterior to the section shown in fig. 4. x 125. 

 p. c, postei-ior commissure ; r. /., Reissner's fibre ; s.c. o., sub-commissural organ. 



The Chimpanzee. — 'Rere., again, we have studied the sub-commissural organ 



by means of serial transverse sections. The entire brain had been hardened 



in situ by means of formalin and alcohol for ordinary anatomical purposes. 



It is naturally not in a good condition for the study of minute histological 



details, but shows very clearly the general form and relations of the parts 



under discussion. 



The sub-commissural organ commences, as in the mouse, on the antero- 

 dorsal aspect of the posterior commissure, where the latter forms the floor 

 of the infra-pineal recess (compare text-fig. 1), where, however, it is very 

 feebly developed. It can be followed backwards beneath the posterior 

 commissure as a broad and very shallow groove (Plate 14, fig. 1, s.c.o.), the 

 epithelium of which shows the nuclei arranged in several layers, but does 

 not exhibit the typical differentiation until we come to about the hinder 

 limit of the posterior commissure (text-fig. 6). Here the sub-commissural 

 organ takes the form of two separate bands of the characteristic high 

 columnar epithelium, which become invaginated into the roof of the iter, 

 just behind the posterior commissure. This invagination (m.r.) turns 

 forwards above the posterior commissure, where it is seen in the transverse 

 section represented in Plate 14, fig. 1, the two epithelial bands having met 



