84 COEFFICIENT OF DIFFUSION OF CELLS 



every way, and each film is always made from a tube 

 containing 10 cc. of this jelly. The substances which 

 are to be tried on the cells are added to the jelly in the 

 form of the Solution 2 (see Chapter III.), which in its 

 turn is added to the Solution 1. The combination 

 (Solution 3) is always in the quantity of 10 cc., and the 

 film is prepared from this. 



It has already been shown that the jelly-film must 

 always contain certain quantities of the salts sodium 

 citrate and sodium chloride, or the cells will not live on 

 it. These salts are therefore added to the jelly-basis or 

 Solution 1. They are added to it in bulk, so that any 

 portion of it contains them, and, in consequence, it is in 

 a condition to cause cells to live on it for as long as 

 possible. 



The jelly is prepared as follows : 



In a beaker of water stand several of the large test- 

 tubes which contain the stock 2-per-cent agar jelly. 

 The amount required will be at least 50 cc. The 

 water in the beaker should be heated until it boils, 

 when the jelly in the test-tube will be melted. 



1 gramme of sodium citrate and then 0.8 gramme 

 of sodium chloride should be weighed out accurately. 

 The two salts are then placed in a flask, which should 

 be of such a size that it also can be steeped in the 

 beaker of boiling water; 49 cc. of the molten 2-per- 

 cent agar solution from the test-tube are now measured 

 out and poured into the flask. The salts slowly dis- 

 solve in the molten agar, and, while this solution is 

 taking place, the flask should be steeped in the boiling 

 water in order to keep the jelly molten. 



