64 PROTOPLASM OF PROTOZOA 



than those of Protozoa, by means of polarized Hght and X-ray diffrac- 

 tion methods, seem to indicate that they are constructed of hpoid and 

 protein molecules, with their long axes arranged perpendicular and 

 parallel, respectively, to the surface of the cell (Schmitt, 1938). E. N. 

 Harvey and Danielli (1936) also hold the cell membrane to be com- 

 posed of lipoid and protein substances. Furthermore, Langmuir has 

 shown that the structure and surface properties of certain nonliving 

 membranes are frequently determined by the orientation of the indi- 

 vidual molecular layers; these may, under certain conditions, undergo 

 almost instantaneous reversal or reorientation. This overturning may 

 markedly alter the chemicophysical properties of the membrane, a fact 

 which may prove to be of considerable interest in the study of the 

 permeability of cells generally. As pointed out by Harvey (1936), one 

 would seem justified in conceiving of the surface membrane in different 

 cells as composed of a monomolecular layer and a polymolecular film, or 

 as a polymolecular oil film with oriented adsorbed protein molecules 

 varying from a rather liquid to solid consistency. 



The remarkable film-forming properties of the ciliate Spirostomum 

 have been studied by Faure-Fremiet, Ephrussi, and Rapkine (1926). 

 This organism explodes when it comes into contact with the air-water 

 interface, and its solid constituents spread over the surface of the water. 

 By first dusting the water with talc, it was determined that the surface 

 film was 4.2 to 5.7 |j p thick, and probably monomolecular. 



PERMEABILITY 



1. Cell membrane. — In Protozoa as in all other types of cells one of 

 the most important properties of the protoplasm is its ability to form 

 new surface membranes which have a selective permeability. In addition 

 to preserving the integrity of the cell, the surface membrane regulates 

 to a large extent the passage of dissolved foodstuffs and oxygen into the 

 cell and the diffusion of waste materials from it. The rate of this exchange 

 depends in part upon the degree of the permeability of the surface mem- 

 brane and in part upon the osmotic concentration of the cell and of the 

 surrounding medium. 



It is not our purpose to discuss generally here the physiological prob- 

 lems of permeability and diffusion. However, many fresh-water Proto- 



