PHYSICO-CHEMICAL INTRODUCTION 



25 



1?=*= 



watci raises the concentration in the cell to the minimum necessary 

 for starch production. 



It has also been shown that unfertilized eggs of the sea-urchin 

 (Strongylocentrotus purpuratus) may be made to develop parthe- 

 nogenetically by the use of hypertonic solutions. 

 The unferti.ized egg of the frog develops if its 

 membrane is pricked with a needle, and its osmotic 

 relation to the surrounding water thus disturbed. 



The Mode of Action of a Semi-Permeable Mem- 

 brane. Since osmosis plays an important part in 

 the maintenance of equilibrium between plant and 

 animal cells and their surroundings, it is highly 

 important to know how semi-permeable mem- 

 branes act. From the study of precipitation 

 membranes, it appears to be the size of the 

 molecular interstices which enables such a mem- 

 brane to differentiate between various substances. 

 It was at first thought to act merely like a 

 sieve, but that is not the sole factor. For 

 example, if a glass tube with a length of rubber 

 tubing and a clamp at the end be filled with 

 carbon dioxide, the rubber then clamped, and the 

 glass tube quickly placed in a vertical position in 

 a beaker of water, the carbon dioxide will gradu- 

 ally diffuse out through the rubber and the water 

 rise in the glass tube (Fig. 15).. Rubber is per- 

 meable to carbon dioxide but not to oxygen and 

 nitrogen. Further, when methyl alcohol and ether y lQ IO._TO SHOW 

 arc separated by a membrane of pig's bladder, th re THE PRINCIPLE OF 

 is an osmotic flow from the alcohol to the ether. 

 If, however, the two fluids be separated by vul- 

 canized rubber, osmosis takes place in the opposite 

 direction. This is because the pig's bladder 

 absorbs ten times as much alcohol as ether, 

 whereas rubber absorbs one hundred times as 

 much ether as alcohol. Therefore the compara- 

 tive permeability or imperm: ability to different 

 substances of a non-living semi-permeable mem- 

 brane depends, also, on its power to dissolve 

 or absorb them. Experiments on living membranes, made chiefly 

 on plants, tend to show that it is a selective absorption on the part 

 of the membrane which determines the ability or inability of a sub- 

 stance to enter the cell. The permeable substances have been found 

 by experiment to be generally soluble in fatty oils; the plasmatic 

 membrane of the cells, therefore, probably consists of some such 

 substance; indeed, it is claimed that cell walls are rich in lecithin and 

 cholesterin both bodies of a lipoid nature. As evidence of this 

 it is found that the basic aniline dyes, which readily permeate 



THE SEMI-PERME- 

 ABLE MEMBRANE. 



diffuses out through 

 the rubber tube at 

 the top, water rises 

 from the beaker, 

 owing to the in- 

 ability of air to 

 diffuse in through 

 the rubber. 



