184 PHYSIOLOGY OF MICROORGANISMS 



colloidal reactions and life-functions. Their bearing has been already 

 indicated (See 168). 



It has already been said that Thomas Graham made the distinction 

 between colloids and crystalloids by means of dialysis through a mem- 

 brane, the colloids are withheld and crystalloids pass through. This 

 movement on the part of these substances follows the laws of diffusion 

 which, in turn, conform with the laws of expansion of gases. In the 

 case where the membrane obstructs the movement of colloids and 

 permits the crystalloids to pass there can be recognized an interference 

 with free movement. Whether the colloidal molecule is larger than 

 the crystalloidal molecule, which appears to be a fairly satisfactory 

 undemonstrated reason, or not, does not materially alter the situation; 

 or whether some chemical transition or obstruction accounts for this 

 phenomenon of passage and check, in our present position, does not 

 contribute much without a real working knowledge of what is involved. 

 The facts remain: Colloidal substances do not pass while crystalloids 

 do. This significant condition may be actually responsible for the 

 cell-entities which incorporate the mechanism of life. 



In colloids, diffusion is slow, slower than in the case of the crystal- 

 loids. This enables the crystalloids to penetrate or diffuse through the 

 colloidal substances as protoplasm and sustain what must be regarded 

 as a more or less fixed substance, protoplasm, through the very nature 

 of its powers. 



The microbial cell is generally a unicellular organism which secures 

 its nutrition and performs its respiratory functions through the surface 

 layer of the cell. This outer layer in most microbial cells takes the 

 form of a membrane and where no membrane exists the cell seems to 

 respond in much the same manner through its protecting surface layers 

 of protoplasm. A yeast cell prepares its food which is not assimilable 

 through its cell-wall by secreting suitable enzymes to produce diffusible 

 nutrition. Such portions of this solution are assimilated through the 

 cell- wall as are needed in cell-construction and are converted by similar 

 processes within the cell substance while in transitional route to 

 protoplasm itself. In the case of an amoeba the particle of food is 

 often taken within the protoplasm by means of its pseudopodia and 

 after digestion is assimilated as in the yeast cell. This process in the 

 amoeba cannot be regarded as at all different from that of the yeast for 

 the digestive-preparatory process and assimilation are much the same. 



