THE VITAL PHENOMENA OF CELLS 



35 



The fact demonstrated by Schiicking in the case of the snail, Aplysia, that 

 long-continued stimulation of the dermal musculature can more than compen- 

 sate the effects of osmotic pressure, and the discoveries on absorption from the 

 alimentary canal (cf. Chapter VIII), together with the facts summarized above, 

 go to show that the osmotic processes cannot be the only factors at work in 

 the absorption of substances by animal cells. The outer limiting membrane 

 of the cell behaves in many respects as a semipermeable membrane, but, so 

 far as we can grasp the matter at present, it appears to differ in many 

 respects from such a membrane. Just as the cell itself regulates the extent 

 of the oxidation processes taking place within it or inaugurated by it, so 

 within certain limits, and independently of the quantitive composition and 

 the osmotic pressure of the surrounding medium, it regulates its absorp- 



FIG. 19. Gromia owiformis, after Max Schultze. Some 

 of the pseudopodia have caught a diatom, which by 

 gradual shortening of the contractile threads will 

 be taken into the interior of the organism. 20/1. 



FIG. 20. Amoeba polypo- 

 dia, after Max Schultze. 

 A small organism has 

 been engulfed. 330/1. 



tion and elimination of substance. It is possible that this is due to the. 

 specific affinities of the living bodies which constitute the protoplasm. Just 

 as gelatin plates and agar plates take up substances from solution, whether 

 the solution is isotonic, hypotonic, or hypertonic, until their affinity for the 

 substance is satisfied, so the living substance might take up or give off sub- 

 stances according to its affinities, independently of the osmotic pressure. Just 

 what weight this hypothesis, developed by Friedlander and Durig, may have 

 we cannot say definitely at present. 



In connection with the investigations here discussed opportunity has often 

 been afforded to determine the osmotic tension inside animal cells. It is found 

 to correspond to a NaCl solution of 0.7 to 0.9 per cent, and amounts to about 

 5 to 6.5 atmospheres. In order that the aqueous content of animal cells may 

 be preserved, it is necessary therefore that the surrounding medium have a 

 corresponding osmotic pressure. 



