IOWA ACADEMY OF SCIENCES 
247 
Intercellular spaces occur in the ependymal tissues of many animals, but 
such cavities are wanting between the epithelial cells of the plexus. 
Gold preparations show that nerves are present in the vicinity of the 
large blood vessels. These are probably vaso motor fibers. 
In regard to capillaries, connective tissues, and endothelium the 
plexuses from other forms show but little variation from what has beer, 
described in the white rat. The greatest differences are found in the 
epithelial cells. In this layer the rabbit differs strikingly from the rat. 
The epithelium of the rabbit is characterized by the presence of clear 
spaces in the cells. They are from 2-6 micra in diameter and very 
numerous. Their contents are dissolved out by elcohol, ether, and xylol. 
In ordinary stained sections they show as clear oval or nound areas. 
The contents are of a fatty nature as can be shown by their staining 
readily with osmic acid and Sudan III. These droplets are largest at 
the apex of the cell. Sometimes one may be seen lying half within, 
half without the cell. This shows that they are expelled through the 
top wall. During this process the nucleus remains normal, but rarely 
it may be pressed in to one side by the droplet. These droplets have not 
been found in the plexus of any other form studied. These clear spaces 
may be seen in the fresh tissue and for this reason it is not believed 
that they can be due to any error in technique. Since their occurrence 
is not general in the forms studied it is not believed that they represent 
the chief secretion of the cells. It is therefore best to consider them as 
of secondary importance. 
A second point in the rabbit’s plexus is the development of the 
parietal structures at the apex of the epithelial cells. In the rat the 
marginal zone is at best but a double contoured line. In the rabbit 
however it is wider and composed of filaments placed perpendicular to 
the surface of the cell and imbedded in some kind of an interstitial 
substance. This gives the cells the appearance of ciliation. The struc- 
ture is what Vignon (1) describes as the “bordure de brosse” or filamen- 
tous plateau. Cytoplasmic microsomes at the base of the filaments 
simulate basal bodies. Terminal bars may be seen in cross section 
at the corner of the cells. 
The plexus of the dog differs little from that of the rabbit except 
by the absence of droplets. The epithelial cells in the guinea pig are 
characterized by a great many nucleoli. 
The preceding description has referred entirely to the adult plexus. 
If a late foetal or newborn specimen be examined striking differences 
will be noticed. The epithelial cells differ from those of the adult in 
three particulars: shape, staining power, and location of the nucleus (2). 
To illustrate, the cells from the plexus of a one day old rat may be com- 
pared with those of an adult. In the one day form the epithelial cells 
are narrower and deeper. Many measurements give the following aver- 
ages. Cells from the one day rat are 13 micra in height and 8 micra in 
(1) . Vignon. Recherches de Cytologie Generale sur les Epithelium. 
Archiv. de Zool. Ex. et Gen. t IX, p. 371, 1901. 
(2) . The attention of the author was first directed to this shift in position 
of the nucleus by Dr. Hatai of the Department of Neurology in the University of 
Chicago. The entire study began from an investigation of that point. 
