62 PROTOPLASM OF PROTOZOA 



functions, such as preserving the integrity of the organism (by being 

 immiscible with water), controlhng the diffusion of materials in and out 

 of the cell (permeability), acting as a seat of electromotive forces (mem- 

 brane or diffusion potentials), maintaining form (possessing tension), 

 forming secretions (for protection, adhesion, forming cyst walls, armor, 

 and so forth) and for the reception of stimuli, and so on. 



STRUCTURE AND ORIGIN 



The problem of surface structure in the Protozoa is often complicated 

 by a failure of many investigators to define clearly what they refer to as 

 the "cell membrane." From a review of the literature it is quite obvious 

 that the vital membrane essential to the cell may be reduced to only 

 a very thin ultramicroscopic film, such as that which forms at the torn 

 surface of an amoeba. In other cases the "cell membrane" is of micro- 

 scopic dimensions and displays the physical properties of a thick, tough 

 pellicle. In this connection the work of Nadler (1929) showed that the 

 pellicle could be completely removed from Blepharisma without killing 

 or even affecting the shape of the organism; after a few days a new pel- 

 licle may be formed in these animals. Could it not be, therefore, that in 

 forms with thick surface layers, the physiologically active vital mem- 

 brane is mainly limited to a thin film, and that the remaining part of 

 the thick surface layer serves mainly for protection, support, giving an- 

 chorage to locomotor organs, secretion of slime for adhesion, forming 

 cyst walls, or in the formation of other surface structures which possibly 

 aid the animal in coping with its environment? 



In addition to the vast literature describing definite morphological 

 membranes at the surface of various Protozoa, both in the living and 

 fixed condition, considerable experimental work has been done on the 

 physical nature of the surface layers. Kite (1913), Chambers (1924), 

 Howland (1924c), Rowland and Pollack (1927), Taylor (1920), 

 Needham and Needham (1925, 1926), and many others have actually 

 established the presence of the surface membrane by puncturing it, tear- 

 ing it, and in some cases actually removing it. In general it has been 

 found to vary in thickness from that of a delicate film to that of a tough 

 pellicle. Microdissection methods have also shown it to possess measur- 

 able elasticity and contractility, and to vary considerably in consistency 

 from that of the underlying protoplasm. 



