520 Prof. A. Dendy and Mr. G. E. Nicholls. [May 24, 



exhibiting a characteristic fold on either side where it joins the former. The 

 cells of the sub-commissural organ are probably provided with cilia, but this 

 is not evident in our preparations. The epithelium 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. 



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 Eeissner's fibre. 



Fig. 4. — Cat. Transverse Section through the Sub-CJoniTnissural Organ in the Region of 

 the Posterior Commissure, x 320, 

 /). c, posterior connni.ssuro ; r. /., Reissner's fibre ; s.c. o., sub-commissural organ. 



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

 it is in tlie mouse, but still very obvious, as will be seen from the transverse 

 section rejireseiited in te.xt-fig. 4. Here the double character of the organ 

 is very well seen ; it is clearly made up of t wo bands of greatly elongated 

 ependymal cells, right and lelX, iind these cells appear to be distinctly 

 ciliated. Reissner's fibre (/••/.), wliich can be traced for a long distance 

 through this series of sections, is clearly seen lying in the groove between the 

 two iialves nf tlic, sub-conmiisHural organ. Text-fig. 5 represents another 

 section lioiii ilu; same series, but near the anterior end of tlio sub-commissural 



