PROTOPLASM 43 



special protoplasmic layer: Strasburger, for instance, believed it to be 

 composed of kinoplasm. 



The microdissection studies of Kite (1913) and Chambers (1917) 

 mentioned above have extended our knowledge of the physical nature 

 of the plasma membrane. Both of these observers describe the ecto- 

 plast of an Amceba as a concentrated gel. Seifriz (1918), as a result of 

 such studies on the Fucus egg and myxomycetes, states that the mem- 

 brane is a definite morphological structure, very elastic and glutinous, 

 and capable of constant repair. He further asserts that membrane form- 

 ation is a physical process dependent upon the physical state of the 

 protoplasm and not upon that of the medium, and that it does not occur 

 after death. 



That the formation of such a limiting membrane at the surface of 

 protoplasm is the result of the tendency of colloidal particles to accumu- 

 late on any interface has been pointed out by physical chemists. Citing, 

 by way of illustration, the film which forms on the surface of cooling 

 milk, Moore (1912) says: "The chief colloid of the milk, on account of 

 its affinities, accumulates on the surface, the accumulation gives increased 

 concentration, the presence of the increased concentration causes the 

 multi-molecules to build together, the larger molecules fall out of solu- 

 tion as particles, and these join to form a close network or film." In a 

 similar manner the unicellular organism or other mass of naked proto- 

 plasm develops its resistant envelope, and the enclosed protoplast of 

 the higher plant its ectoplasmic layer and tonoplast. 



Permeability. The physico-chemical nature of the plasma mem- 

 brane has been a subject of much discussion among physiologists. On 

 the assumption that the permeability of the cell is a case of solubility in 

 the ectoplasm, E. Overton (1895, 1899, 1900) developed a theory of the 

 constitution of the ectoplast. It was pointed out first, that the ectoplast 

 is not miscible with water; second, that in plant and animal cells the only 

 bodies which are not miscible with water in the ordinary state are fats and 

 oils; third, that the ectoplast is more or less permeable to substances ac- 

 cording as the latter are more or less soluble in fats and oils; and fourth, 

 that any substance insoluble in another substance will not pass through a 

 membrane composed of the latter. It was therefore concluded that the 

 ectoplast is made up of some lipoid compound, such as lecithin, which 

 acts as a semi-permeable membrane. This theory, though very sug- 

 gestive, was effectively opposed by Ruhland (1909, 1915) and a number 

 of other investigators, who called attention to many substances which 

 do not behave according to the requirements of the theory stated in 

 so simple a form. A more nearly adequate conception of the constitution 

 of the ectoplast has thus been souhgt. 



Of the more recent theories which have been offered in connection 

 with the problem of permeability the most promising are those which 



