500 



cilia, but this is not evident in our preparations. The epitheliuna 

 shows a strong tendency to split longitudinally at the bend of the 

 horseshoe, which possibly indicates its paired character. A further 

 indication of this paired character is seen in the section represented 

 in text-fig. 3, where the epithelium is cut through quite separately 

 and in two almost detached patches, as it projects from the anterior 

 surface of the posterior commissure. 



i^i%,i^^»'ij*; 





Fig. 3. Mouse. Transverse section through the region of the superior (habe- 

 nular) commissure at about the level of the line cd in text-fig. 1. )x( 36. 



c.c. corpus callosum ; Ji.g. habenular ganglion ; i.p.r. infra-pineal recess ; s.c. 

 superior commissure ; s.c.o. sub-commissural organ ; s.p.r. supra-pineal recess ; v. veins. 



It is thus evident that in the mouse the sub-commissural organ 

 is a very conspicuous structure, which probably has some important 

 function connected with the associated Reissner's fibre. 



The Cat. — In the cat the sub-commissural organ is not so well 

 defined as it is in the mouse, but still very obvious, as will be seen 

 from the transverse section represented in text- fig. 4. Here the 

 double character of the organ is very well seen; it is clearly made 

 up of two bands of greatly elongated ependymal cells, right and left, 

 and these cells appear to be distinctly ciliated. Reissner's fibre 

 {r.fX which can be traced for a long distance through this series of 

 sections, is clearly seen lying in the groove between the two halves 



