COMPARATIVE STUDY OF MEMBRANE PERMEABILITY 629 



It is obvious from the results reported here, that the distribution of the 

 permeabihty characters in the small intestine is different from that in the 

 colon or in the frog skin. The organization and the distribution of the 

 permeability characters is thus an important aspect of cell differentiation. 

 A purely speculative scheme has been proposed to explain the possible 

 evolution of the permeability characters in the epithelial tissues ([7], p. 136). 

 According to this hypothesis the primitive state should be characterized by 

 a mechanism of active transport located at both inner and outer border of 

 the cell. The cell would then evolve toward an asymmetrv bv losing the 

 active transport mechanism at one of the borders. The results reported 

 here give some experimental support to this conception. 



It is also worth noting that the small intestine and colon are differ- 

 entiated from the same embryonic layer, the endoderm, while the frog 

 skin comes from the ectoderm. Cells originating in two different embry- 

 ologic layers (colon and frog skin) may thus evolve in the same direction 

 (convergent differentiation). 



Without going further into a detailed comparative studv of membrane 

 permeability it is apparent that epithelial cells, such as those of frog skin 

 and turtle intestine, possess in principle the same permeability characters 

 as those found in conducting cells, such as electroplax, nerve fibre or 

 muscle, or those found in red blood corpuscles. But one advantage of the 

 epithelial cells studied in the present paper lies in the fact that some of the 

 permeability characters, e.g. passive permeability to Na and K in frog skin 

 and turtle colon, are spatially separated at a microscopic level or are found 

 together in the same membrane (turtle small intestine), while in the nerve 

 fibre for instance, one of them, the Na selective character, appears only 

 for short periods of time (action potential). 



Further investigations are now necessary to answer the question 

 whether or not the characters of passive permeability to Na and K are 

 spatially separated in the small intestine cells. One attempt to solve this 

 problem has been made with the isolated electroplax of the electric eel 

 Electrophonis electricus L. and the reader is referred to another publication 

 for a more thorough analysis of this matter [7]. 



Another interesting point raised by the above considerations is the 

 question of the chemical nature of the molecular architectures responsible 

 for the various permeability characters. One may obtain information on 

 this subject by studying the effect of various classes of compounds known 

 to affect the permeability of living membranes. It is a well-known fact that 

 ouabain and some ammonium derivatives affect ionic movement in a 

 variety of cells functionally different or belonging to species situated far 

 apart on the evolutionary scale [7 19]. These results suggest that a com- 

 mon biochemical system could be responsible for all the permeability 

 characters ot living membranes. This would thus mean that we could 



