42 CELLULAR STAINING, DEATH, ACHROMASIA 



which runs up into it. The amount drawn into the 

 tube can, if necessary, be controlled by keeping the 

 finger on the other end. It has been found most service- 

 able to allow the solution to fill the tube to the extent 

 of about half an inch, and any excess can always be 

 removed by tapping the lower end of the tube upon the 

 table, which causes some of it to run out. Having got 

 a sufficient quantity of citrated solution into the tube, it 

 is run down to one end of it, and a mark is made at the 

 upper limit (or meniscus) with a grease pencil. The 

 fluid is now run along the tube by depressing its other 

 end until its lower meniscus stands at a level of the 

 mark, and a second mark is then made at the upper 

 meniscus, after which the tube is again placed vertically 

 so that its contents runs down to its original position. 

 The finger having been pricked, a drop of blood is 

 squeezed out and at once allowed to run into and mix 

 with the citrated solution in the tube, the greatest care 

 being taken that no air-bubble intervenes between the 

 fluids. The blood should be allowed to run in until 

 the upper meniscus of the mixed fluids reaches the 

 upper mark. Thorough mixing of the blood with the 

 citrated solution is ensured by rocking the tube in such 

 a way that its contents runs from end to end. The 

 mixture in the capillary tube will now consist of equal 

 proportions of blood and citrate solution, and of this a 

 drop is tapped out on to a cover-glass, which is then in- 

 verted and allowed to fall on the agar film in the usual 

 way. When tissues are to be examined, a small portipn 

 of the growth or normal structure is either teased 

 out or scraped into a little of the citrate solution in 



