68 AN INTRODUCTION TO THE STUDY OF VIRUSES 
appear aggregated without the same regularity into microclumps in 
which the particle separations seem to be about twice their normal 
value (Black, Price, and Wyckoff, 1946). 
When a suspension of typhus rickettsiae is extracted with ether, 
a “soluble” antigen may be liberated which has many of the immuno- 
logical properties of the rickettsiae themselves. This antigen, whose 
active principle will pass the usual bacteria-proof filters, is highly 
active in the complement fixation and precipitation reactions. Electron 
micrographs of a concentrated suspension of typhus or Q-fever 
rickettsiae show them to be enmeshed in a thin membranous material 
suggestive of bacterial capsules. Extraction with ether at room 
temperature does not alter the appearance of the rickettsiae them- 
selves, but it does profoundly affect their capsules which are broken 
up and, in a sense, emulsified. It appears, therefore, that the “soluble” 
antigen of typhus and probably of other rickettsiae consists of sub- 
microscopic particles of a capsular substance (Shepard and Wyckoff, 
1946). 
Another interesting and important application of the electron 
microscope is the study of induced changes such as the action of the 
bacterial viruses upon bacteria. Such a study should yield information 
as to how viruses develop and multiply within their host cells, and 
bacteria, as Wyckoff (1948) has pointed out, are very suitable for this 
purpose as they are small enough to be partly transparent in the 
electron microscope. 
Wyckoff (1947° and 1948) has studied the lysis of E. coli by the 
T strains of bacteriophages. The protoplasm of the lysed bacteria has © 
a characteristic fine structure with an extraordinary degree of regu- 
larity. It consists of elements, about the size of bacterial virus particles, 
which usually appear as concavities and are frequently lined up in 
rows or regular networks throughout the protoplasmic mass. Wyckoff 
has also recently shown that filaments, as well as spheres, are present 
in purified influenza virus suspensions and that an intimate relation 
seems to exist between the two forms. 
If plant viruses multiply by binary fission, then it should be possible 
sooner or later to photograph the virus particles in the act of dividing. 
Examples of such fission are rarely seen in the routine photography 
of purified virus solutions but if it were possible to photograph the 
virus inside the living cell, it may be that dividing particles of viruses 
would be more frequently seen. 
It is a characteristic of viruses that they mutate and some electron 
