156 THE PRINCIPLES OF IMMUNOLOGY 



bacilli as outlined above may be mixed with a drop of i per cent, 

 neutral red in isotonic salt solution. At first the extracellular bacteria 

 show no stain, and but few of the intracellular bacilli take the stain. 

 As time goes on the number of intracellular organisms taking the stain 

 increases until they are completely digested. Metchnikoff interpreted 

 the coloration of the bacteria as being due to an acid digesting fluid 

 formed by the cell, but we are unable to state at the present time 

 whether the digestion of the bacteria is due to a special ferment or 

 due to the same ferments that digest the cells themselves after they 

 are destroyed. 



The Physical Basis of Phagocytosis. The mechanism of phagocy- 

 tosis both as regards immunity and biology in general has been the 

 the subject of much investigation. There are those who have main- 

 tained that the ameba or the leucocyte, in spite of the absence of a 

 nervous system, exhibits individual intelligence in the selection of the 

 material it takes up, but the bulk of experimental evidence would place 

 the phenomenon largely on a physical chemical basis. There are im- 

 portant differences between free living amebse and the phagocytes of 

 higher animal life, such as the ectosarc and endosarc of the amebse, 

 its pulsating vacuoles, variety of pseudopodia, conjugation and cyst 

 formation, but there are resemblances in movement, form, nutrition and 

 ultimate genesis which form a basis for many comparisons. That the 

 life activities of the ameba can be closely simulated by non-living 

 materials has been known for many years, but the most important 

 stimulus to these studies in recent years has been given by the work 

 of Jennings. A fundamental conception necessary to understanding 

 the physical basis of ameboid motion and phagocytosis is that of the 

 phenomena of surface tension. Wells expresses the matter most 

 clearly and concisely as follows : " Imagine a drop of fluid suspended 

 in water let it be a drop of protoplasm, or oil, or mercury ; the drop 

 owes its tendency to assume a spherical shape to the surface tension, 

 which is pulling the free surface toward the center and acting with the 

 same force on all sides. The result is that the drop is surrounded by 

 what amounts to an elastic, well-stretched membrane, similar to the 

 condition of a thin rubber bag distended with fluid. If at any point 

 in the surface the tension is lessened, while elsewhere it remains the 

 same, of necessity the wall will bulge at this point, the contents will flow 

 into the new space so offered and the rest of the wall will contract; 

 hence the drop moves toward the point of lowered surface tension. 

 Conversely, if the tension is increased in one place the wall at this 

 point will contract with greater force than elsewhere, driving the con- 

 tents toward the less resistant part of the surface, and the drop will 

 move away from the point of increased tension." The experimental 

 demonstration of this phenomenon is relatively simple. A drop of mer- 

 cury is placed in a nitric-acid solution and near it is placed a crystal of 

 potassium dichromate. A yellow color diffuses out from the dichro- 

 mate; as the color reaches the mercury the latter begins to move 

 toward the crystal. This is the result of oxidation of the adjacent 



