156 III. THE EVOLUTION OF VIRUSES 



time the demonstration of the presence of the virus becomes impos- 

 sible (64). It is a note-worthy fact that the demonstration of the 

 virus alvi^ays becomes difficult in this way, when tumours undergo 

 cancerous transformation. This may indicate that the structure be- 

 comes weaker as it changes into cancerous one. 



Such a weakness of cancerous structure may be based upon its 

 juvenile and primitive nature. As will be fully discussed in Part V, 

 there are sufficient reasons for supposing that non-differentiated, juve- 

 nile protoplasm structures are generally weak in their template action, 

 so that they may be readily assimilized by other stronger, more dif- 

 ferentiated structures. As is well known, chick embryos are exten- 

 sively applied to cultivate various viruses, a fact which may be based 

 upon their juvenile character to be readily assimilized by other proto- 

 plasm structures. In a similar way, it has been ascertained with a 

 number of viruses that cancer cells can sustain the multiplication of 

 virus like chick embryos (64 a) (64 b). 



Cancer cells are distinct in their embryonic nature. Although the 

 cells having the embryonic nature are generally able to proliferate 

 vigorously when existing as intact cells, their specific protoplasm 

 structure may be lost because of its unstable character when the cells 

 are decomposed into fragments. Even when the structure was re- 

 tained in the fragments, the particles would be unable to act as the 

 virus because of its inability to act as the template on account of its 

 Aveak pattern. 



The ftiain reason why cancers are regarded as one of the most 

 dreadful diseases may lie in their faculty to form metastasis in remote 

 parts of- the^body^ It is generally believed that for the metastasis the 

 intact cells "-are indispensable. But it may be conceivable that the 

 fragments^, of the cells may also transmit the structure, since the 

 fragments can enter the blood-vessels in their fresh state and can 

 reach, immediately after their liberation, healthy organs or tissues 

 where prevails the high predisposition to the cancer. 



Virus diseases are called as such because they are transmitted by 

 protoplasm particles of the diseased cells, and hence they are regarded 

 as infectious diseases, whereas ordinary tumour diseases, such as Rous 

 sarcoma and Shope papilloma, cannot be called infectious, although 

 they are sometimes transmissible by virus-like agents. They are not 

 infectious because the infection cannot take place in natural ways, a 

 fact which may depend upon the unstableness or the weakness of proto- 

 plasm structure of the tumour cells. As a rule the structure appears 

 to be preserved only in the intact cells. Accordingly the disease can 

 be transmitted generally by the intact cells, not by their fragments, 

 and hence the structure cannot exist independently of the living intact 



