504 



coming bent back upon itself for about 0.06 mm. to its blind termin- 

 ation. 



The diameter of the recess, as seen in text-fig. 9, is only about 

 0.02 mm., the recess being in this case much longer than wide, in 

 fact, distinctly tubular. The lumen of the tube is somewhat triangular 

 in transverse section (text -fig. 9), widening suddenly to its opening 

 behind (text-fig. 8). 



The elongated, radially arranged cells lining the recess are about 

 0.02 mm. in length. 



Behind the recess the ependyma of the sub - commissural organ 

 passes gradually into the general ependyma of the iter. 





Fig. 8. 



Fig. 9. 



Fig. 8. Human foetus of about five montlis. Transverse Section through the 

 sub-commissural organ at the posterior end of the posterior commissure, y^ 115. 

 e.p. e. ependyma! epithelium of the iter. s.e. o. sub-eommissural organ. 



Fig. 9. Human foetus of about five months. Transverse Section through the 

 mesocoelic recess, slightly anterior to the section represented in text-fig. 8. X ^'^0. 



m.r. mesocoelic recess, s.c.o. sub-commissural organ, forming the epithelial lining 

 of the mesocoelic recess. 



In sections through the region of the posterior commissure in the 

 brain of the adult human subject the sub-commissural organ is seen 

 to have almost entirely vanished. The ependyma covering the ventral 

 surface of the commissure consists apparently of somewhat flattened 

 cells with conspicuous nuclei, and differs very little from the general 

 lining epithelium of the rest of the iter; the only difference being that 

 upon the side walls of the iter the nuclei are more crowded, and 

 possibly the epithelium slightly more cubic or columnar, but much less 



