EXPERIMENTS ON FIXATION 315 



Allow the gross sediment to fall. Decant the fluid and centrifuge 

 it to get rid of the rest of the sediment. 



Put the supernatant fluid in a flask and add 25 ml of 10% acetic 

 acid to precipitate the nucleoprotein. Warm gently to 37^ and 

 leave in an incubator at this temperature for 15 minutes to com- 

 plete the precipitation. 



Shake the flask and pour the contents into a measuring-cylinder 

 or other tall, narrow vessel. Leave for i| hour. Pour off the super- 

 natant fluid and discard it. 



Centrifuge the wet sediment and discard the supernatant fluid. 



Wash the sediment by adding 150 ml of 0-25% acetic acid, 

 stirring, and re-centrifuging. 



Repeat the washing twice (three washes altogether). 



The sediment is wet nucleoprotein. It may be dried in a desicca- 

 tor over calcium chloride, but the fully dried substance is rather 

 troublesome to re-dissolve and it is best to proceed as follows. 

 Weigh a gram or two of the wet sediment accurately and desiccate 

 to constant weight. The loss in weight will show how much of the 

 wet substance must be taken to make a nucleoprotein solution of 

 any particular concentration. The wet substance may lose about 

 I of its weight on drying. 



Preparation of gelatine / nucleoprotein gel 

 Put 15 g of powdered gelatine in a beaker and add 50 ml of potas- 

 sium hydroxide, 0-25% aqueous solution. Place in an incubator at 

 37° C and stir occasionally until the gelatine dissolves. This is 

 solution A. 



Put some wet nucleoprotein containing 4 g of dry nucleopro- 

 tein in a measuring cylinder. Add 17 ml of potassium hydroxide, 

 10% aqueous solution. Stir. Make up to 50 ml with distilled water. 

 Place in an incubator at 37° C and leave till w^arm, with occasional 

 stirring. This is solution B. 



Mix A and B, stirring well. Leave in the incubator for J hour, 

 stirring from time to time. The solution may then be sucked into 

 pipettes while still warm, for experiments on the rate of penetra- 

 tion of acetic acid (p. 39). It hardens to a gel on cooling. The gel 

 may be kept temporarily in a refrigerator. 



The coagulation of egg -albumin by various fixatives 

 Add to egg-white 4 times its volume of distilled water ; mix ; clear 

 the solution by centrifuging. The albumin will now be present at 

 about 2-6%. 



