III. THE ACTIVITY OF VIRUS PARTICLES 23 



the virus actitity is relatively low or when the resistance of the host 

 cell is relatively high, more than one particle are necessary to infect 

 a single host cell. In addition, there are many reasons to suppose that 

 the degree of protoplasm change is directly proportional to the quantity 

 of agents causing the change (46) as will be shown later where a discus- 

 sion is made on the mechanism of haemolysis. 



3. The Activity of Vaccinia Virus 



The vaccinia virus, with which we carried out a series of experi- 

 ments, did unfortunately not exhibit a high activity upon rabbit skin. 

 The number of infectious particles present in a virus sample isolated 

 by us was so small that the ratio of the number of infective particles 

 to that of the total was found, when examined with rabbit skin, to be 

 of the order of 1 : 1,000. Nevertheless, it may not be proper to conclude 

 from this fact that our virus sample was highly impure. The skin of 

 the rabbits which we employd showed individually different susceptibi- 

 lity to the virus, that is, the susceptibility varied with the individuality 

 of the rabbit. Therefore, it might be possible to change the ratio 

 if we made experiment with other rabbits more susceptible to the virus 

 or with some appropriate tissues other than the rabbit skin. 



Nakamura and Ohfuji (52) found that a minute amount of the vac- 

 cinia could much easier be detected when injected into rabbit testicles 

 than when injected into its skin ; they proved under the microscope the 

 existence of the infection in the testicles by the histopathological changes. 

 Nakamura and Fukumura (53) have also confirmed that the infective titre 

 of the emulsion of cells which had been infected with the vaccinia virus 

 which they possessed, as measured by the intracutaneous injection into 

 rabbit, was 1x10* to 1x10^, while as measured by the above method 

 of the intratesticular injection, 1x10^ to 1x10^. Consequently, if the 

 infective titre of the virus particle, all of which were assumed to be 

 active in the testicles, was measured by the intracutaneous injection, 

 it would appear, as a matter of course, as if only one particle amongst 

 1,000 to 10,000 were the true virus. It is, however, not reasonable to 

 consider that all strains of vaccinia are always so highly infective to 

 testicles. For example, since the vaccinia isolated by Smadel et al (54) 

 was claimed to be so effective on the skin that almost each particle 

 was able to produce the infection, it cannot be expected that such a 

 vaccinia is so much more infective to testicles. On the contrary, the 

 existence of strains of vaccinia may be possible which is more in- 

 fective to skin than to testicles. Moreover, the existence of vaccinia 

 particles which can neither exhibit their action in testicles nor in skin 



