524 



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



iter, forming a ])air of small shallow grooves, as shown in text-fig. 7, 

 Laterally these grooves are not clearly marked off from the general 

 ependyma, which, as is well known, is much more strongly developed in- 

 the foetus than in the adult, but is distinguished from the epithelium of the 

 grooves by the much more superficially-placed nuclei. Mesially the edges 

 of the grooves become more sharply defined (text-fig. 7), the ependyma having 

 practically disappeared- from beneath the commissure between the grooves 

 (compare cat, text-fig. 5). 



Towards the hinder end of the posterior commissure these two grooves 

 unite again and become invaginated into the roof of the iter to form a 

 mesoccelic recess exactly comparable to that already described in the 

 chimpanzee, but more widely open below (text-fig. 8). 



This recess is lined throughout by the characteristic ependyma of the sub- 

 commissural organ, and tunnels forwards into the brain tissue above the 

 postei'ior commissure for a distance of about 0'35 mm. Its full length is. 

 rather greater than this owing to the tube becoming bent back upon itself for 

 about 0"06 mm. to its blind termination. 



Fio. 8. — Human foetus of about five riiontli.s. Transverse Section through the Sub- 

 commissural Orffan at the I'osterior End of the .Posterior Commissure, x 154. 

 e.p. e., ei)en(lymal epithelium of tlie iter ; s.c. o., sub-commissural organ. 



The diameter of the recess, as seen in tcixt-lig. 9, is only about O'OIJ mm., 

 the. recess being in this case much longer than wide — in fact, distinctly 

 tuljular. The lunicn of the, tulic is soiiiewhut triangular in transverse section 

 (tfxt-fig. 9), wiilcning suddenly to its oiieiiiiig licliind (lext-fig. 8). 



