VI. THE SHAPE OF VIRUS PARTICLE 85 



are commonly caught well in micrographs. 



On the one hand, it seems generally accepted that nucleic acid 

 contents are increased when protein synthesis js to develop vigorously 

 for the rapid multiplication of the cells ; on the other hand, there is 

 a good reason to consider, as will be detailed in a later chapter, that 

 nucleic acids are capable of stabilizing the structure of protein poly- 

 mers and thereby the character of elementary bodies for acting as a 

 virus is intensified. The production of a tail in the phage particle 

 may also be attributed to the high content of nucleic acid, which may 

 cause some proteins to be left in a stretched form. 



As already mentioned, virus-like particles are actually absent in 

 the normal skin tissue, although virus particles may develop in it on 

 the infection with a certain virus. Virus-like particles can be produced, 

 however, in the skin tissue when a proper stimulus other than viruses 

 is applied. Therefore, even when particles never present in norm.al 

 tissues are found in some pathological tissues, it cannot be said that 

 the particles have a definite connection with viruses. The success of 

 Angulo et al. referred to above, in demonstrating various virus-like 

 particles may be attributed to their investigation with cells undergoing 

 a change by the infection with the microorganism which may develop 

 a change like that induced by a virus. 



Epestein et al. (94) have reported that between 24 and 72 hours 

 after inoculation of the virus of foot-and-mouth disease into guinea 

 pigs, within which interval generalization of the disease reached its 

 height and the red blood cells were found infective for normal guinea 

 pigs, particles with mean diameter of 246 m// were found in the red 

 cells by the electron microscope. They claimed that the particles were 

 absent in normal control material. However, their particles appear 

 to be similar in size to those indicated in Fig. 12. If there was no 

 mistake in their demonstration of the particles solely during certain 

 periods after the infection, it should be interpreted as that the parti- 

 cles become during the periods stable enough to reveal their complete 

 figures under the condition of their examination. 



According to electron microscopic studies of Wyckoff (2) the pro- 

 toplasm of young actively growing cells of E. coli docs not contain 

 filaments or particles but old cells commonly have the spherical macro- 

 molecules embedded in a matrix of filaments. There are many good 

 reasons to believe that the structure of proteins becomes, as a rule, 

 more stable and rigid as they grow older, a full account of which 

 will be given later in Part V. 



Under the electron microscope, materials to be examined must be 

 brought into an unusual dried state because of the vacuum, and at the 

 same time must be exposed to a high temperature by the bombard- 



