INTRODUCTORY 3 
bacteria per c.c., only a few miles farther down the river less than a 
thousandth of that number could be found. Obviously something 
was destroying the bacteria. In 1915 Twort noticed that clear patches 
developed in colonies of bacteria growing on an agar plate as if the 
organisms were being “eaten up.’ Being interested in “filterable 
viruses,” Twort passed some of the culture showing the clear spots 
through a bacteria-proof filter candle and added some of the filtrate 
to a fresh culture of bacteria. He found that the filtrate was capable 
of producing clear patches in that culture also. This demonstrated 
that something of a virus nature was destroying the bacteria. A year 
or two later d’Herelle, who was investigating a bacterial disease of 
locusts, discovered the same phenomenon and called it the bacteriophage 
(bacterium eater). Now the study of the bacteriophages, or bacterial 
viruses as they are called at the present time, constitutes an important 
branch of virus research. 
As far back as 1911 the American worker Peyton Rous demon- 
strated that a malignant new growth in a fowl could be transmitted 
by means of a cell-free filtrate. This was the first demonstration of 
the fact that viruses can cause tumours and may yet prove to be a 
discovery of profound significance. In 1928 Purdy showed that 
tobacco-mosaic virus was a potent antigen and that when inoculated 
into a rabbit it gave rise to a specific antiserum. This discovery opened 
the way to a great deal of intensive work on the serology of plant 
viruses. 
In 1931 Elford developed his new series of graded collodion mem- 
branes, called “‘gradocol’? membranes, which have been much used 
for filtration work with viruses and for measuring their particle size. 
Beginning at about this period, and from time to time since then, 
some new plant viruses have been described, the study of which, 
apart from their intrinsic interest, has yielded results of great scientific 
importance. Among these viruses may be mentioned that of tomato 
bushy stunt, the tobacco necrosis viruses, and the virus of turnip 
yellow mosaic. In 1933 the discovery that influenza was due to a 
virus was made by Smith, Andrewes, and Laidlaw, and later came the 
development of the technique for growing this and other viruses on 
the chorio-allantoic membrane of the developing hen’s egg (Burnet, 
1935). 
In 1935 came the isolation, by Stanley, of the virus of tobacco 
mosaic in the form of fine needles or paracrystals, an achievement 
which has stimulated much chemical work on viruses. Shortly after 
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