46 AN INTRODUCTION TO THE STUDY OF VIRUSES 
A method of obtaining highly purified preparations of an animal 
virus by means of centrifugation alone has recently been described by 
Taylor (1946) and the following description is taken from his account 
of his work. The virus in question is that of the Shope rabbit papilloma 
which produces large warty outgrowths on the skin of infected cotton- 
tail rabbits. The sources of virus were the naturally occurring papillomas 
obtained from rabbits trapped in Kansas, U.S.A. 
The warts were removed from the animals in the field, covered with 
a mixture of glycerol and o-9 NaCl in 8-oz bottles and stored at a 
temperature of 2-8°C. The growths were collected over a long 
period and stored until sufficient material was obtained for the work. 
In a typical experiment, 150 g of warts were freed of hair and ex- 
traneous tissue and broken up in about 100 ml of o-9 per cent NaCl 
solution. The pulp was made up to a volume of 21 (a 7-5 per cent 
tissue suspension) and set aside at 2-8°C to extract overnight. The 
suspension was then mixed with 50 g of No. $12 celite and filtered 
through a thin mat (2-3 mm) of No. $03 celite. The resulting, slightly 
turbid, filtrate was passed into the Sharples supercentrifuge through a 
No. 24 gauge hypodermic needle at a constant rate of $00 ml per hour. 
A centrifugal field of 62,000g was maintained at the inside bowl 
periphery (50,000 r.p.m.). The extract was followed with 500 ml 
of a solution containing 0:13 M NaCl and 0-05 M phosphate buffer, 
pH 6:5, to displace the final 50 ml of extract and to wash thesedimented 
virus. As the speed of the bowl slowed, the lower inlet boss was 
stoppered. 
The bowl, containing the concentrated virus in a volume of 50 ml, | 
was removed from the machine and put in the cold room overnight. 
The next morning, after thorough shaking of the bowl, the 50 ml 
of virus suspension were washed into a 100 ml centrifuge tube with 
20 ml of the buffer saline solution. This was spun in an angle centrifuge 
for 15 minutes at 3000g and the supernatant fluid decanted. The 
soft, insoluble sediment was washed twice with 25-ml portions of 
buffer saline solution, and the wash fluids were added to the initial 
concentrate. The virus recovered from an initial volume of 21 of 
crude extract was now in a volume of 120 ml. This was next subjected 
to a single cycle of centrifugation in the vacuum-type air-driven 
ultracentrifuge (sooog for 5 minutes, 50,000g for 1 hour), after which 
the pellets were taken up in 6:5 ml of the buffer saline solution. Large 
ageregates were eliminated by low speed centrifugation in the vacuum 
ultracentrifuge at 5000g for $ minutes. 
