living leucocytes in vitro. 



31 



fixed on a clean slide, so as to wall in a small chamber, with an 

 entrance passage leading into it, thus : 



A drop of blood is allowed to fall into the chamber at A, 

 a coverslip is superimposed and gently pressed down with a glass 

 slide, so that as the plasticine is flattened, blood and air are driven 

 out of the passage B, which must be kept patent, so that the 

 chamber is completely filled with an even layer of blood, thus : 



This needs a little practice to perform successfully, but under 

 the usual circumstances, a small air bubble in the chamber does 

 no harm. 



The chamber is now incubated, at about blood temperature, 

 for any length of time, from ten minutes to three or four hours, 

 according to requirements. 



For a class of students a very simple arrangement, doing away 

 with the need for an incubator, is as follows. A warm stage is 

 prepared under a microscope, by means of a copper strip, one 

 end of which, resting on the microscope stage, is kept at blood 

 temperature (the melting point of a fragment of cocoa butter can 

 be used as a rough indicator), when the other end is heated by a 

 very small flame ; on the end above the flame, a flat white dish, 

 filled with normal ('75 °/^) saline solution, is placed, at a point on 

 the copper (easily found by experiment) where the saline solution 

 also keeps warm at a temperature of 38° C, or a little below. 

 The whole slide, on which the chamber has been prepared, is 

 immersed in the dish of saline, at blood temperature, and so 

 incubated for at least ten minutes. 



During the period of incubation the blood clots and the 

 leucocytes escape and adhere firmly in hundreds to the surface 



