170 L. M. BLACK 



kno-^ai volume of inoculum at a high dilution, usuaUv 10~^. He calculated 

 that the dilution of the starting quantity of virus at each transmission was 

 10~* because the inoculum injected into each insect was approximately 1/10 

 its weight. After 10 passages in series, the dilution had reached lO"***, whereas 

 ordinarily the virus from insects is infectious at dilutions of 10~^, but not 10~*. 

 In fact, Maramorosch's starting dilution in this series, 1/7000, was the greatest 

 dilution of aster yellows virus successfully inoculated up to that time. In 

 retrospect, it seems probable that the high dilution of his inocula, as well as 

 the optimum temperature at which the experiment was conducted, were 

 important in the success of his experiment. This experiment proved that 

 aster yeUows virus multiplies in its vector. Maramorosch later (1955) demon- 

 strated that this virus could be passed serially from vector to vector when the 

 insects were maintained only on insusceptible plants. 



Maramorosch told the WTiter of his success at an early stage m the conduct 

 of his first experiment on serial passage of the virus through the vector and an 

 attempt of the same sort was started with wound tumor virus and its vector 

 Agallia const ricta. The insects used as a continuing source of virus were 

 always kept on immune plants. At each passage a sample was tested on 

 susceptible plants and then discarded. The results (Black and Brakke, 1952) 

 confirmed completely Maramorosch's conclusions that a plant virus could be 

 transmitted in series from vector to vector without loss of titer mider condi- 

 tions which assured no replenishment of virus from plants. Similarly, the 

 virus of corn stunt was transmitted at dilutions of 10"^ from vector to vector 

 through 3 serial passages (Maramorosch, 1952b). 



In a critical review of the subject. Black (1953b) considered in detail the 

 controls that were employed in various experiments up to that time and the 

 interpretations and conclusions that had been made. Relevant objections 

 that had been raised were considered in detail. The results of Fukushi's 

 experiment were re-evaluated in the mamier employed for the treatment of 

 the data on clover club leaf; it was thereby demonstrated that the dilution of 

 virus in the female with which Fukushi started his experiment was at least 

 1/563,000 and probably at least 1/10^ in the sixth generation of progeny. 



At that time. Black (1953b) estimated that the number of virus particles 

 probably carried in a leaf hopper ranged from a minimum of 10^ to a maximum 

 of 10^2. It is now possible to give an estimate of the average number of 

 particles of one virus, namely, wound tumor, that occurs in its vector. The 

 titer of this virus in a group of infective leaf hoppers was measured serologically 

 by titrating the virus zone from rate density gradient centrifugation tubes. 

 The end-point titer was about 1/320, or 10"^-^ (Black and Brakke, to be 

 published). The best estimates of the diameter of the virus give a value of 

 75 m/x. This is based on measurements of electron micrographs of the purest 

 preparations of the virus obtained (Brakke et al., 1954). The volume of the 



