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IOWA ACADEMY OF SCIENCES 
foetus shows the plexuses as large swollen glandular organs occupying 
practically all the space in the lateral ventricles. Along with this 
anatomical fact has grown the idea that the plexuses furnish some kind 
of a fluid food necessary to the brain during the embryonic period. 
Accordingly it has sometimes been called the “cerebral placenta”. What- 
ever the case may be in man it seems that the above description does 
not apply to all forms. In the w’hite rat the size of the plexuses has 
been carefully studied at various ages. There is a steady gradual 
growth from the time of the first invagination until the adult appearance 
is reached. At no time is there any evidence that the organ is enlarged 
or distended, or that it fills anything like all the ventricular space. 
From a freshly killed animal the plexuses may be removed and 
examined in cerebro-spinal fluid or in normal salt solution. Not a great 
amount of detail can be obtained in this way, still it serves as a control 
for the sections subjected to reagents. The cells appear uniformly gran- 
ular with nuclei shadowy and cell boundaries indistinct. The most 
careful examination has failed to show the presence of cilia in the adult 
forms. They are present in young animals immediately after birth, but 
it seems reasonably sure that cilia are not present in the adult. At the 
same time cilia are often noticed on the ependyma which has been torn 
from the ventricular walls. 
Under the influence of fixatives and stains many other details appear. 
The cross section of a small loop of an adult rat’s plexus shows the 
following structures. In the centre of tth section are one or more capil- 
laries. These consist of a delicate endothelial intima which is strength- 
ened by connective tissue cells and fibrils. Between this adventitia and 
the epithelium are more connective tissue cells and their processes. The 
epithelium itself is composed of cubical cells which in cross section 
average about 10-12 micra in width and 8-10 micra in height. Basal and 
lateral walls are poorly defined but may usually be distinguished as 
hazy lines. The free edge of the cell is slightly convex. It consists of a 
thin apical plate or cuticle. The cytoplasm is finely reticular. In the 
meshes are deeply staining granules which are often collected into 
small irregular masses. The reticulation for the most part is more pro- 
nounced near the periphery. The nuclei are circular in outline and cen- 
trally located. 
Embryologically the epithelial cells are derived from the inner layer 
of the neural tube which also produces the ependymal cells. The 
ependymal and epithelial cells are therefore parts of the same layer, 
and this may be plainly seen by tracing the plexus back to where its 
epithelial covering joins the ependyma lining the ventricle. The two 
types of cells pass into each other by an easy gradation. The epithelial 
cells have lost all projections from the base which is so characteristic 
of the ependymal cells. The epithelial layer has also lost all traces of 
the neuroglia which normally overlies the ependyma. 
The epithelium of the plexus consists of but a single layer of cells. 
In pathological conditions there may be a proliferation and even a 
stratified condition, but in normal tissue this seems never to be the case. 
