505 



distinctly so than in the foetus. The ependyma beneath the posterior 

 commissure is continuous round the anterior surface of the latter with 

 that which lines the infra-pineal recess, and in no wise differs from 

 it. There is thus upon the surface of the commissure nowhere any 

 indication of that characteristic elongatedly columnar epithelium that 

 marks the sub- commissural organ in lower types. 



At the hinder limit of the posterior commissure, however, about 

 the middle line, there is an irregular thickening of the ependymal 

 epithelium, and above this an irregular cavity occurs in the tissue of 

 the brain, lined unevenly by the remains of an epithelium, with num- 

 erous scattered nuclei, exactly like that on the roof of the iter beneath 

 it. No opening could be found putting this cavity into communication 

 with the cavity of the iter, but numerous scattered nuclei connect the 

 two epithelia, and suggest the remains of such a communication. 



In successive sections this irregular cavity can be traced forwards 

 and upwards, behind and above the posterior commissure. It obviously 

 corresponds to the similarly situated mesocoelic recess in the foetus, 

 although lacking its characteristic high columnar ependymal lining. 

 Anteriorly it again becomes choked with scattered cells, forming an 

 ill-defined tract which passes forwards and then abruptly gives place 

 to a small, but very well defined, sub - spherical chamber (v. Plate, 

 fig, 3, m.r.). This chamber, moreover, is lined by exactly the same 

 type of elongated epithelium as characterises the sub -commissural 

 organ in the foetus and in lower forms. We regard this chamber as 

 simply the blind anterior termination of the mesocoehc recess (com- 

 pare Plate, fig. 1, and text- fig. 9). The lumen here has a diameter 

 of about 0.05 mm., while the cells of the lining epithelium are about 

 0.02 mm. long, with the nuclei placed as usual towards their inner 

 ends. In the lumen is a small quantity of coagulum. 



In the brain tissue around the posterior portion of the mesocoelic 

 recess (i. e. that portion where the cavity has become irregular and 

 largely obliterated) occur numerous small globules (v. Plate, fig. 2, gl.) 

 which have stained brightly red in the preparations (which were treated 

 with borax carmine in bulk, followed, upon the slide, by picro-indigo- 

 carmine). Whether these are due to pathological or to post-mortem 

 changes we are unable to say; that they are normally present seems 

 improbable — in any case they are conspicuous in these sections, and 

 interesting as occurring closely adjacent to the last vestiges of the 

 sub-commissural organ. 



We have thus found the sub-commissural organ fairly well de- 

 veloped in the human foetus, and in an entirely vestigial condition in 



