40 INTRODUCTION TO CYTOLOGY 



involved. In the Arhacia egg there is a reaction of calcium with a sub- 

 stance visible in the cytoplasm, according to Heilbrunn (1928, 1930a). 



Because of the very different surface conditions prevailing in tissue 

 cells, the differentiations can scarcely be expected to be wholly the same 

 as in "naked" masses of protoplasm. Nevertheless, there is reason to 

 believe that here, too, specialized osmotic membranes are responsible for 

 certain physiological phenomena, notably selective permeability, as was 

 claimed long ago by Pfeffer (1890). Plowe (1931a6) has been able to 

 show both by microdissection and by permeability studies that the plasma 

 membrane in the familiar cell of the onion-bulb scale is a real structure. 

 That surface membranes have properties not present in the endoplasm is 

 shown by the fact that certain substances, eosin for example, to which the 

 membrane is impermeable, will diffuse readily through the endoplasm if 

 artificially injected through the membrane (Chambers). Such semi- 

 permeable membranes (membranes permeable to solvent but not to 

 solute) are also present about nuclei and sap vacuoles (see below). All 

 such membranes of the protoplast appear to be lyophilic gels. In the 

 case of a vacuolate cell, the permeability of the whole is determined by the 

 permeabilities of all the regions traversed — plasma membrane, endoplasm 

 and vacuole membrane — but it is due chiefly to the membranes. 



Because of the great importance of the cell membrane in physiology 

 many studies have been made with the hope of determining its exact 

 physico-chemical constitution.-^ One of the most influential theories has 

 been that developed by E. Overton (1895 et seq.), who concluded that the 

 membrane is composed primarily of lipides. This theory was founded 

 chiefly on the high correlation found to exist between the lipide solubility 

 of many organic compounds, notably dyes, and their ability to pass 

 through cell membranes. IMoreover, lipides would be expected to 

 accumulate at free surfaces, and their presence would help to explain the 

 immiscibility of the membrane with water. This theory has been much 

 criticized, not only because it has been thought not to account satis- 

 factorily for the high permeability of the membrane to water, but also 

 on the basis of further studies on permeability and leaching.-^ 



The probable importance of proteins in the cell membrane has been 

 emphasized by a number of investigators, ^^ partly because of the colloidal 

 nature of these substances and the readiness with which they form pre- 



2^ For discussions and literature pertaining to the constitution and physiological 

 behavior of plasma membranes, see Schaeffer (1920), Bayliss (1921), LundegSrdh 

 (1922), Hober (1922), R. S. Lillie (1923), Chambers (1924), Jacobs (1924), Stiles 

 (1924), Weber (19266), Gellhorn (1929), Steward (1929), Gray (1931), and 

 Beutner (1933). 



2^Ruhhmd (1908 et seq.), Hober (1909, 1922), Steward (1928a6, 1929), Bailey 

 and Zirkle (1931). See, further. Gray (1931). 



26 Pfeffer (1900), Ramsden (1904), Osterhout (1911), Loeb (1911), T. B. Robertson 

 (1908), Lepeschkin (1910, 1911). 



