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



together and completely surrounded the recess thus formed. This recess ends 

 blindly in front at a distance of about 0'225 mm. from the point where it 

 turns forwards above its opening into the iter. It has an internal diameter 

 of about 0'25 mm. The enormously elongated, almost fibre-like cells by 

 which it is surrounded measure about 0"057 mm. in length, and their nuclei 

 are situated towards their inner ends. Though not in a good state histo- 

 logically, it is evident that they agree closely in character with the 

 corresponding cells in the cat and mouse. 



Fio. 6. — Chimpanzee. Transverse Section through the Sub-Commissural Organ at the 

 Posterior End of the Posterior Commissure, x 60. 

 m. r., mesocffilic recess ; s.c. o., sub-commissural organ. 



This recess, which forms a very conspicuous feature in transverse sections 

 of tlie chimpanzee's brain, corresponds in position with what Sargent (1904) 

 has termed the rcr.rs.ms mrsocoelims in the sea lam})rey {Prtromyzon marinus), 

 where also he regards it as part of the sub-commissural organ (Ependymal 

 Groove). Stieda long ago (1870, fig. 15) figured a similar recess above the 

 posterior commissure in the frog's brain, but without recognising its sig- 

 nificance. Gaupp (1891), fig. 17) also figures this recess in the frog, and 

 terms it the " diverticuluin imj)ar." 



We have already noticed in the mouse a tendency on the ])arL of the 

 epithelium of the, sub-comniissural organ to become invaginated l)i'iiin(l the 

 jjoaterior conimi.s.sure, so that tliis tendency would a2)pcar to bo very general 



