MICROSCOPIC METHODS 



83 



to which has been added a little of the blood of a person suspected to 

 have typhoid. Note these points in Fig. 58: the organisms are evenly 

 distributed throughout the field, except at the edge of the drop, where 

 they are gathered in great numbers; they show great activity here, 

 seemingly trying to crowd to the very edge. This attraction is probably 

 due to the action exerted on the organisms by the oxygen in the air, 

 which naturally exerts positive chemotaxis on all aerobic organisms. 



Fig. 59 shows a cross-section of the drop represented in Fig. 58, and 

 it will be noticed that the bacilli are evenly distributed throughout the 

 drop, except at one place in the focal plane a, and again in the focal 

 plane c. 



It sometimes happens that there is a substance adhering to a sup- 

 posedly clean cover-glass which attracts the bacilli to that point, where 



FIG. 60 



FIG. 61 



Microscopic field, showing the top of a drop with 

 the typhoid reaction. 



Microscopic field, showing a cross-section of 

 the drop in Fig. 60. 



they appear as fairly well-defined clumps, more or less like the true 

 clumps due to the agglutinating substance in typhoid blood. The 

 increase in organisms at the bottom of the drop in the focal plane c is 

 easily accounted for by the fact that gravity naturally carries the dead 

 and non-motile organisms to the bottom, these frequently assuming the 

 character of clumps. 



If a field can be found in any focal plane of the hanging drop free 

 from clumps, one can be quite sure that any clumping present is not 

 due to any agglutinating substance which necessarily will affect 

 organisms in every focal plane. 



Fig. 60 shows the microscopic appearance of the top of a drop 

 where the reaction is present. Notice first that the organisms have been 

 drawn together in groups and that the individuals of each group appear 

 to be loosely held together. Viewed under the microscope these clumps 

 are practically quiescent, there being very little movement either of the 

 individual organisms or of the clump as a whole. The edge of the drop 



