II. THE GENERATION OF VIRUSES 149 



This peculiar seasonal chang-e in the human impregnation may be 

 based upon certain metabolic activities mainly concerning with hor- 

 monal secretions, upon which the season may have great influences. 

 It may be a reasonable explanation that the same seasonal influences 

 are involved in the morbility change of measles, and the change in 

 the susceptibility to the disease due to season may account for the 

 phenomenon. However, if the susceptibility fluctuate regularly with 

 season, measles virus should be present everywhere, always, and in 

 abundance, in order that the susceptibility curve was precisely repre- 

 sented by the morbility curve, since the elevation of susceptibility 

 would not be followed by the increase in the morbility unless the 

 virus was present everywhere and in abundance. Can this actually be 

 possible ? As the time seems not ripe to discuss furhter of this pro- 

 blem, it will be left untouched until we have a proper occasion later. 



In order to show how complicated is the problem concerning the 

 virus generation, an experiment carried out by us (44) in connection 

 with phage production will be discussed below. As above stated, 

 phage is excreted from some animal cells, while flies appear likewise 

 to excrete phage. 



According to our investigation, common house flies almost con- 

 stantly contained large amount of phage. However, phage could never 

 be detected in the flies which had been raised from eggs in rearing 

 boxes. If large amounts of phage were fed to such flies with foods, 

 only small portions of the phage were found in the flies and even the 

 detectable phage itself would disappear in a few days. It was quite 

 impossible to make them carry phage as in naturally living flies. In 

 addition, naturally living flies carry the virus only in summer, a fact 

 comparable to the phage detection in chicken feces only in this season, 

 suggesting that the virus is also excreted by the insect as by other 

 animals. 



Thus it was supposed that some factor or factors capable of stimu- 

 lating the phage production might be lacking in the reared flies. We 

 were, however, unable to find the supposed factor, although a variety 

 of kinds of foods were given under various conditions in taking into 

 consideration such physical factors as temperature and sun-light. 



Though we failed to arrive at any definite conclusion on this pro- 

 blem, our finding may present some suggestions as to the origin of 

 common cold. As stated above, for the occurrence of common cold the 

 predisposition and the stimulus, /. e., chilling, appear to be necessary, 

 but according to English workers (48), chilling neither induces nor 

 favours common cold. The writer is inclined to think that such an 

 unexpected conclusion, obviously contradictory to every man's personal 

 experiences, has been arrived because of the isolated, unnatural living 



