ENERGY CHANGES INVOLVED IN SECRETION 271 



the dissolved molecules themselves, they may be caused to move 

 in a definite direction and afr a definite speed through the cell, 

 different from that of the water in which they are dissolved. A 

 similar directive movement, in fact, to that seen in the case of 

 dissolved ions and colloidal molecules in the electric field may occur. 



Or the energy changes may be brought about by chemical 

 combinations and dissociations in the cell. 



But whatever view be taken as to the mode of operation, it 

 is clear from the experimental study of the selective rates of passage 

 of dissolved substances through the cell that what might be termed 

 " polar " properties must be ascribed to the cell in its phenomena of 

 secretion and absorption. This is not theory but experimental fact. 

 It is seen that many substances pass through the cell several times 

 more rapidly than the solvent, while others pass through more 

 slowly. In the case of those which pass through more rapidly, 

 work in giving velocity to these molecules or ions and in increasing 

 osmotic energy must be done by the cell. It is clear from this that 

 the amount of solvent in which any given quantity of a constituent 

 is dissolved need never enter the cell, but instead the dissolved sub- 

 stance may be attracted and moved through the solvent towards and 

 into the cell by the energy of the cell, just as independent velocities 

 are given to the ions towards the electrodes by the electric poten- 

 tials on the electrodes without the solvent between the electrodes 

 moving at the same rate towards either electrode. Thus there 

 may only be a slow current of water through the cell, with a slow 

 uptake of water from the lymph, and a much more rapid current of 

 dissolved substances and corresponding increase in concentration 

 of these in the secretion or absorbed fluid. 



If the water containing the dissolved substances were taken 

 up at the same rate by the secreting cell, then in order that the 

 secretion could become more concentrated in any constituent 

 it would be necessary that at intervals water should be returned 

 or pressed out again at the side of the cell at which it entered, 

 containing the constituent which was to be concentrated in more 

 dilute solution. It hence appears more probable that instead 

 of such a to-and-fro movement of water, the dissolved substances 

 are taken up upon the entrance side of the cell more rapidly than 

 if they passively moved in with a corresponding amount of water. 



That this view is probable is seen from the enormous amount of 

 water which would have to pass into and out of the cell alternately 



