STOCK SOLUTION OF AGAR 39 



chloride, and 2-per-cent sodium citrate. 1 (When this 

 jelly is spread on the slide it will contain 1-per-cent 

 agar, 0.9-per-cent sodium chloride, and 1-per-cent 

 sodium citrate.) The picture presented by blood spread 

 on such a film is very different from those in the last 

 two experiments. The red cells are not crenated, but 

 are beautifully spread out. The leucocytes are not 

 dead, but alive and amoeboid ; no Brownian movements 

 of the granules can be seen, and the cells do not burst; 

 on the contrary, they will live now for an hour or more. 

 It may therefore be said that for the examination of 

 living blood-cells (and it has been found that it is also 

 the case for all cells yet tried) the jelly must always 

 contain a certain amount of the salts sodium citrate 

 and sodium chloride. "Normal saline" is not enough 

 by itself. Cells die immediately when they are resting 

 on a surface which contains only sodium chloride. 



These three experiments will prove instructive for 

 the beginner with this jelly method, for they demon- 

 strate how the jelly is prepared. It must be observed 

 that for the purposes of these researches the supply 

 of jelly is always kept as a 2-per-cent solution of agar. 

 When, however, it is placed as a film on the slide, it is 

 always diluted with an equal volume of some other 

 solution, so that the film invariably contains 1 per cent 

 only of agar. It is in the diluting solution (always 

 added in an equal volume) that the salts, and any other 

 substances to be experimented with, are contained, and, 

 obviously, before being added to the agar they must 

 be of twice the required strength so as to be reduced 



1 Potassium oxalate may be substituted for sodium citrate. 



