32 I. INTRODUCTION TO VIRUSES 



be converted into the particles. This is also the case with normal 

 particles having no connection with virus. 



The size of virus particles is mainly determined by the kind of cells 

 from which they have been derived. Vaccinia virus particles isolated 

 from the calf or rabbit skin is evidently different in size from those 

 isolated from the rabbit testicle, the former being much larger than the 

 latter. Again, phage particles are even much smaller than the vaccinia 

 virus isolated from the testicle, while the normal bacteria are disinte- 

 grated into particles of a small size entirely similar to that of phage. 

 It seems probable that viruses, not only in size but also in immunolo- 

 gical and even in chemical properties, are sujected to the cells from 

 which they were produced. 



The water quantity combining with the protoplams particle is 

 estimated to be so great as approximately ten fold of its dry weight, 

 no matter whether the particle is produced by a virus or not. 



Usually virus particles consist of a protein and lipids like the 

 normal protoplasm. This may only be a natural result if virus particles 

 are nothing but the particles produced by coagulation of the protoplasm. 



The number of active particles in a phage sample is revealed to be 

 different with the condition under which the number is measured even 

 when the same strain of host bacteria is used. If various kinds of 

 host bacteria are used the difference becomes very striking. This may 

 be attributed to the different phage-susceptibility of the bacteria, 

 varying with the environmental condition as well as with the strain. 



A strain of bacteria with an extremely high susceptibility for a 

 phage may be infected under a very suitable condition by a single 

 particle of the phage, though usually more than one particle seem to 

 be required for the infection. This holds true also for other viruses 

 such as vaccinia and some plant viruses. 



In addition, each virus particle may have each individuality and 

 accordingly even in one and the same sample, particles with various 

 properties may be involved. Therefore, the estimation of the absolute 

 titre of a virus 8ample seems to be extremely difficult, if not entirely 

 impossible. 



It is worthy of note that a cell generally tends to be disintegrated 

 into similar sized particles whose size seems to be determined by the 

 kind of the cell. However, there are usually some differences in size 

 even among the virus particles produced from the same kind of cells, 

 and smaller particles prove to be much more unstable than larger ones. 



