144 C - M - CHILD. 



in rate of penetration of permanganate, in consequence of dif- 

 ferences in permeability of different regions of cell or body, or 

 whether they are more directly related to metabolic conditions 

 in the protoplasm. With respect to this question it may be 

 noted first that the color gradient begins to appear as a super- 

 ficial gradient, apparently resulting from the deposition of 

 MnO 2 on the cell surface. This is particularly evident in single 

 cells, e. g., Noctiluca, Slentor, eggs and blastomeres, where in 

 optical section it can be seen that no appreciable penetration 

 has occurred when the gradient has already begun to appear. 

 Of course it cannot be denied that in such cases the permanganate 

 has already penetrated a membrane of molecular thickness, but 

 it certainly has not penetrated the visible structural membrane 

 of the cell. In the protozoa the gradient is wholly or almost 

 wholly limited to the ectoplasmic layer, but in eggs it apparently 

 exists to some degree throughout the cytoplasm but appears 

 first superficially. The facts seem at least to indicate that dif- 

 ferences in a purely physical permeability of the cell membrane 

 will not account for the color gradient any more than for the 

 susceptibility gradient or the gradients in differential inhibition 

 and acceleration of development. 



As regards the question of permeability, however, it is doubtful 

 whether we can isolate it as a purely physical condition from the 

 chemical activity going on in the living protoplasm. Recent 

 investigation indicates more and more clearly that such isolation 

 is impossible. Without going into the matter at length, it is of 

 interest to note that Osterhout has conceived changes in per- 

 meability in terms of chemical reaction, that R. S. Lillie in his 

 later papers has emphasized the importance of metabolic reac- 

 tions in relation to permeability and that recently Crozier ('i8a, 

 &) has concluded that for acids and alkalies the essential factor 

 in stimulation is not an increase in physical permeability by 

 depolarization, but rather a chemical reaction between the agent 

 and some constituent of the receptor cell. And finally, even if 

 we accept a purely physical theory of permeability and assume 

 that the axial gradients in susceptibility to various agents and 

 rate of staining by permanganate represent primarily gradients 

 in permeability, there can be no doubt that such relatively per- 



