IOWA ACADEMY OF SCIENCES 
249 
In the rat experiments with muscarin did not give decisive results 
but in the case of rabbits and guinea pigs the results were definite. 
Often as many as two-thirds of the cells showed evidences of secretion. 
Normally the epithelial cells of the rabbit are about 6 micra high . but 
after the injection of muscarin the height increases to 12 micra. A 
differentiation into an outer clear zone and a basal granular zone is 
rather well marked. Granulations are heavier toward the base of the 
cell. Clear spaces begin to appear toward the top and rarely does the 
stainable cytoplasm extend to the upper cell wall. Large masses of 
granules occur in the upper part of the cell where the lines forming 
reticulations cross. The nucleus remains globular with a clear outline. 
The things most striking about these modified cells are their great 
increase in height and the appearance of so much clear space at the 
apical end. 
Parietal structures are not supposed to take any part in the functional 
activity of epithelial cells. In case of the extrusion of droplets they 
may be opened but there is no change in their structure during the pro- 
cess. In the case of the rabbit where there are fatty droplets which 
we consider a secretion of secondary importance, we can find no evidence 
of any change in the marginal zone of the cell. In the case of the nor- 
mal secretion, however, the evidence seems to be that the marginal zone 
of the cell is modified. The apical wall seems to decrease in thickness 
as the cell increases in height. We do not believe that it disappears 
during the secretion but it evidently is considerably modified. 
The fact that the fiow of the cerebro-spinal fiuid is increased by the 
injection of drugs that produce secretory phenomena in the epithelial 
cells, justifies the conclusion that the fiuid is secreted by the choroid 
plexuses. No doubt the ependymal cells may have their part in the pro- 
duction but it is certainly a minor one compared to that of the plexuses. 
Additional evidence that strengthens this conclusion is the occurrence of 
hypertrophy of the plexuses in certain cases of hydrocephalus. The 
fact that the fiuid differs from serum or lymph is also evidence that is 
in favor of this view. 
