40 AN INTRODUCTION TO THE STUDY OF VIRUSES 
virus diseases of insects, and compared with the virus diseases of other 
types of organisms this branch of virus work has been much neglected. 
There is a good deal of evidence for the existence of other virus diseases 
attacking the larval forms of insects in which the characteristic poly- 
hedral bodies are not formed, their place being taken by large numbers 
of very small granules. The exact significance of these granules 
is not known. 
Before concluding this brief survey of representative virus diseases, 
mention must be made of the bacterial viruses or “bacteriophages” 
as they used to be called. 
The destruction or “‘lysis” of bacteria by some agent which appeared 
to be of a virus nature was first observed in 1915 by Twort. He made 
the chance discovery that colonies of micrococci growing on agar as 
a contaminant from glycerolated vaccine virus tended to become 
“slassy” or transparent. Sub-cultures from such colonies grew for a 
time and then became “glassy” in their turn (upper photograph, 
Plate VIII). 
Twort considered that some virus-like agent must be concerned in 
the destruction of the bacteria since filtrates of “glassy” material 
diluted 1 : 10,000,000 were still active. Two years later d’Herelle 
described a similar phenomenon in growing cultures of B. dysenteriae. 
Since that time the bacterial viruses have been most intensively 
studied, especially in the U.S.A., and a vast literature on the subject 
has grown up. Since this chapter is concerned only with the des- 
cription of virus diseases and not of the viruses it will be relevant to 
describe some effects of the viruses on the bacteria. 
d’Herelle had observed that soon after the exposure of bacteria to 
virus, the former begin to swell and finally burst, due to internal 
pressure which he thought was exerted by the parasites multiplying 
within the cells. A study of the intimate mechanism of the lysis of 
bacteria by the virus was made by Bronfenbrenner, Muckenfuss and 
Hetler (1927), who took a cinematographic record of lysis of B. coli 
on agar at 37°C. Bronfenbrenner describes the process as follows— 
The progress of lysis was photographed by means of an automatic camera 
at a rate of 25 exposures per minute. The record shows that, after a short 
period of lag, bacteria began to multiply at a rate notably exceeding that 
of normal bacteria photographed under similar conditions but in the absence 
of phage (virus). This stimulating effect of phage has been reported by a 
number of investigators. Due to the rapidity of growth, many cells failed 
to divide completely and gave rise to filaments having a length of from 
10 to 20 times that of an average normal cell. By the end of the first hour 
