410 PERMEABILITY IN PLANT AND ANIMAL CELLS 



them being partially filled with vaseline.^ The disks thus prepared 

 were used in the same way as the disks of Laminaria. The condi- 

 tions were therefore strictly comparable. In order to make this 

 doubly sure, pieces of Laminaria of the same size as the pieces of frog 

 skin were fastened between disks of hard rubber in the same manner 

 and used for comparison. The resistance of a pair of disks containing 

 frog skin was in the neighborhood of three times that of a pair of disks 

 containing Laminaria. Usually five pairs of disks with frog skin 

 were placed together to form a cylinder whose resistance was 

 measured.^ 



The measurement of the resistance of the frog skin was less accu- 

 rate than that of Laminaria. The point of minimum sound in the 

 telephone was more indefinite (especially with live tissue) . The re- 

 sistance of the control was not constant as in the case of Laminaria. 

 The controls of frog skin were placed in sea water + four volumes of 

 distilled water, which was taken as approximately isotonic (for con- 

 venience this will be called 0.2 sea water). In this solution the re- 

 sistance sometimes remained constant for some time, but more often 

 it rose somewhat and finally became constant for a time or else began 

 slowly to fall.^ Increasing or diminishing the proportion of distilled 

 water did not help. Soaking the frog in the solution for an hour 

 before removing the skin made no decided difference. In spite of 

 these difficulties it was possible to select from several lots of material 

 some whose resistance did not change much in 0.2 sea water, and thus 

 to obtain consistent results. It was found desirable to express all the 

 net resistances as per cent of the controls.'' 



" Cf. Osterhout, W. J. V., /. Biol. Chem., 1918, xxxvi, 557. The method shown 

 in Fig. 7 was usually employed. The apparatus described as type B was also 

 used and gave similar results. 



* Five pairs of disks containing Laminaria had about the same resistance as a 

 cyhnder of Laminaria tissue consisting of 80 pieces with only one rubber disk at 

 each end. 



^ Regarding changes in the conductivity of frog skin, under various conditions, 

 see Hober, R., Physikahsche Chemie der Zelle und der Gewebe, Leipsic, 4th edi- 

 tion, 1915, 441. 



"^ For example, a piece of frog skin was placed in NaCl and after a given time the 

 resistance was measured; this was divided by the resistance of a control in 0.2 

 sea water measured at the same time (both lots having been placed in the solution 

 at the same moment) . In 0.2 sea water the skin remained alive for several hours. 



