STUdCTlTKE AND DEVELOPMENT OE REISSNEU’S EIDRE. 21 
nerve-fibres situate at the junction of the fore- and miij-brain. 
Tliis tract, which is known as the posterior commissure, is by 
most autliors considered as belon^inj; to the mid-brain, of 
which it is said to mark the anterior bonndar}'. It makes its 
nppearance in development at an extremely early stage, in 
tliat downfolding of the roof (the plica meso-prosence- 
phalica^) which separates the first brain vesicle from the 
second. Arising at tlie snme time, or perhaps even earlier in 
development, is another equnlly constant but little-known 
structure, the “ sub-commissural organ.” This is a con- 
spicuous, longitudinal, paired tract of epithelium, produced 
b}’^ a modificalion of the ependymal epithelium of the brain 
ventricle on either side of the mid-dorsal line beneath the 
meso-prosencephalic fold. The ordinary, almost cubical 
cells of the ependymal epithelium become in this region 
enormously elongated and fibre-like. Their nuclei mostly 
pass inward towards the end remote from the brain ventricle, 
and the whole structure bears a striking resemblance to the 
epithelium of a sense-organ. From the inner (deeper) end 
of the cells neuroglial fibres pass, which, collecting into 
bundles, radiate towards the membrana limitans externa 
on the upper surface of the brain. The ventricular ends of 
the cells ai-e beset with short cilia. 
In the adult, in almost eveiy case, the ependymal epithelium 
of the sub-commissural organ j)asses gradually, in the infra- 
])incal recess, into the more typical epithelium of the 
epiphysial stalk. 
The paired nature of this sub-commissural organ remains 
apj)arent throughout life in certain forms (e.g. Petromyzon- 
tidte), but in the greater number of cases there is a con- 
fluence of the two tracts along their mesial borders, so 
that the structure takes on the form of a median plate of 
columnar cells, and under such circumstances may retain 
traces of its originally paired character only at its anterior 
and posterior ends, d'he shape of this plate is variously 
‘ Plica iiieso-thalamenceplialica would seem to he a better term 
for this fold. — A.D. 
