JOHN W. CHURCHMAN 21 



without the addition of a mordant. Pure mordants have a strong chemical affinity 

 both for the substrate and the dye, and are used where an anchorage of dye in sub- 

 strate is desired. 



Decolorizers are used to withdraw stains from certain tissues or organisms or 

 parts of an organism and thus by a process of "regressive staining" to differentiate 

 them. "Regressive staining" is contrasted with the ordinary method of "progressive 

 staining" in which the process is stopped when only those substances with great af- 

 finity for the dye are stained. 



STRUCTURE OF THE CELL 



To appreciate the significance of staining phenomena and to be able to discuss the 

 mechanism involved it is not only necessary to understand the structure of the dyes 

 but also to be familiar with present-day conceptions of the structure of the living cell. 

 The cell is not to be thought of as a mere random mixture of cell constituents. These 

 constituents have a permanent spatial distribution and physical state. This special 

 structural constitution or organization is responsible for the special peculiarities of 

 chemical behavior. Present evidence indicates that the basic protoplasmic structure 

 has a closer resemblance to an emulsion type of structure than to that of any other 

 simple physical system. The reactions which cells undergo proceed most actively — 

 although perhaps not exclusively — at the boundaries of protoplasmic phases; in other 

 words, the surfaces of membranes, fibrils, granules, and other solid cell structures have 

 an accelerating or catalytic influence on these reactions. Many features of the chem- 

 ical organization and behavior of living protoplasm appear to depend on the presence 

 of thin films (apparently consisting chiefly of lipoid material) by which its structural 

 elements are bounded and inclosed. The entire cell is inclosed by a thin, semiperme- 

 able film — the plasma membrane; the internal protoplasm is probably partitioned by 

 similar films. Apparently the intracellular partitions undergo increase of permeability 

 or break down at death so that many chemical reactions which are absent or inappre- 

 ciable during life proceed rapidly in dead cells. The type of structure characteristic of 

 living protoplasm appears to be one by which free diffusion is prevented or restricted. 



Various theories have been advanced to explain cell permeability, most of which, 

 though not all, presuppose a plasma membrane which exhibits differential properties 

 permitting some substances to enter the cell easily (alcohol, ether) ; others with diffi- 

 culty (most salts, sugars, etc.); still others not at all (most colloids). While not uni- 

 versally accepted, the membrane hypothesis seems to be the one with which the known 

 facts are best in accord. In the theory of Overton this membrane is supposed to be 

 impregnated with lipoids, probably not ordinary fat but a mixture of lecithin and 

 cholesterol. Others (Ramsden, Loeb, Crozier) emphasize the importance of protein 

 in the membrane, the first calling the structure a "haptogen membrane" ; and Nathan- 

 sohn postulated a membrane composed of a mosaic of both lipoids and proteins. The 

 size of the molecule doubtless plays some part in penetrating power; and if molecular 

 weight be very high, penetration is prevented. But it must not be thought that be- 

 low a certain point there is any correlation between molecular weight and difficulty of 

 penetration; indeed, the reverse is often the case; fatty acids apparently enter cells 



