170 III. THE EVOLUTION OF VIRUSES 



In order to form a particle larger than an elementary body of the 

 protoplasm, a certain number of elementary bodies have to associate 

 with one another to form a larger body, since the size of elementary 

 body cannot be altered, and hence highly developed viruses including 

 Rickettsiae have to be composed of a number of elementary bodies. In 

 fact, this has been shown in the electron micrographs of Rickettsiae 

 and of some large sized viruses. 



2. Indisputable Organisms 



The tact that Rickettsiae can be resolved microscopically by visi- 

 ble light, and that they are held back by membranes which allow 

 most of the filtrable viruses to pass through, may bring them into 

 line with the bacteria. However, their general failure to grow on or- 

 dinary culture media, and their predilection for intracellular multipli- 

 cation, show that they are more akin to filtrable viruses. 



It is, however, a fact worthy of note that although none of the 

 pathogenic species of Rickettsiae has ever been cultured apart from 

 living cells, the commensal species, R. melophagi, found in the sheep- 

 ked, is said to be able to grow on blood agar (88). 



The ability to grow without living cells may seem the indication 

 of the appearance of mysterious nature, peculiar to living organisms 

 and lacking in viruses. Nevertheless, there seems no need to postulate 

 the appearance in this Rickettsiae of a mysterious nature, because the 

 ability to grow without living cells may be interpreted as to be the 

 faculty to produce its replica in blood proteins as well as in proto- 

 plasm proteins, whilst viruses and usual Rickettsiae are merely in 

 the position to produce their replicas only in the protoplasm. 



In addition, in various respects, blood may be looked upon as a 

 large pool of a fluid protoplasm. If acetic acid is added to blood 

 serum diluted with water, virus-like particles, composed of euglobulin 

 and lipids, will be yielded as in the case of cell protoplasm treated in 

 a similar way as was already stated in Part I. Moreover, intimate 

 mutual association seems to exist among protein molecules in blood as 

 in the protoplasm. In the protoplasm, owing to this mutual associa- 

 tion among protein molecules, various changes, including coagulation 

 process, can spread readily. In like manner, in blood the coagulation 

 process can spread successively as will be mentioned later. 



The mechanism, therefore, by which the Rickettsiae can accom- 

 plish the multiplication in blood agar can be regarded as being similar 

 to that whereby viruses multiply in the protoplasm. However, since 

 in its physical and chemical properties blood differs considerably from 



