I should like to have your comment on the point raised by Dr. Abler. It seems to me that the question is not that of the relative tonicities of CSF and plasma. Of course these fluids are in osmotic equilibrium. The real question concerns what happens to red blood cells suddenly removed from plasma and dumped into CSF. These cells suddenly find themselves surrounded by a now-hypertonic environment and consequently they lose water. However, I see no reason to doubt that existence of a special specific factor which carries an electrical (?) charge important in maintaining the concave disc.