502 



separate bands of the characteristic high columnar epithelium, which 

 become invaginated into the roof of the iter^ just behind the posterior 

 commissure. This invagination {m.r.) turns forwards above the posterior 

 commissure, where it is seen in a transverse section (v. Plate, tig. 1), 

 the two epithelial bands having met 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 sur- 

 rounded 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. 



This recess, which forms a 

 very conspicuous feature in trans- 

 verse sections of the chimpanzee's 

 brain, corresponds in position with 

 what Sargent (1904) has termed 

 the recessus mesocoelicus in the 

 sea lamprey (Petromyzon 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 significance. Gaupp 

 (1899, fig. 17) also figures this recess in the frog, and terms it the 

 "diverticulum impar". 



We have already noticed in the mouse a tendency on the part 

 of the epithelium of the sub-commissural organ to become invaginated 

 behind the posterior commissure, so that this tendency would appear 

 to be very general throughout the vertebrate series. It is possible 

 that in all cases the invagination may be apparent rather than real, 

 and due to overgrowth of the sub-commissural organ by the sur- 

 rounding tissues. 



We have found no trace of Reissner's fibre in the chimpanzee, 

 but it was hardly to be expected in material preserved merely for 

 anatomical purposes. 



Fig. 6. Chimpanzee. Transverse section 

 through the sub-commissural organ at the 

 posterior end of the posterior commissure. 

 X36. 



m.r. mesocoelic recess; s.c.o. sub-com- 

 missural organ. 



