CLONAL PHENOMENA 



filamentous form. After long passage they take on what we 

 regard as the normal spherical form. Recently we studied 

 this process directly and found that the same rules held. If 

 a filamentous strain is passaged by the limit-dilution method, 



Fig. I . A diagram to illustrate the function of limit-dilution titration. 

 Each set of bars represents the composition of a population containing 

 a dominant form D and a mutant from this, M. The first and second 

 pairs represent two possible results of inoculating only D into the medium 

 or susceptible tissue. If the second population is diluted io~^ the 

 amount transferred in passage would be equivalent to the area above Ay 

 that is, ten infective doses of Z), none of Af. The resulting population will 

 be shown in the next set A. If a dilution of i to i oo only is used for passage 

 (B) both D and M will be transferred, and if M has a significant pro- 

 liferative advantage it will become the new dominant form as in the 

 final set B, 



it Stays filamentous. Repeated passage en masse gradually 

 allows the replacement of the long forms by spheres. We are 

 incHned to think that the production of filaments is an indica- 

 tion that the virus is not yet properly adapted to its new host. 

 When the right mutations occur toward better adaptation, 



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