30 JOHN H. NORTHROP 



after inoculation with virus. All of these workers agree that the bacterial 

 cell must be in good physiological condition and also that each cell forms 

 a certain definite amount of virus. 



Delbriick and Luria (1942) have investigated the reaction in a series 

 of extremely careful and interesting papers and have obtained a detailed 

 picture of the steps in the increase of virus. Their results may be sum- 

 marized as follows : 



1. Virus is rapidly absorbed by the host cell, the rate of absorption 

 being proportional to the concentration (of. Krueger, 1931). 



2. No increase in infective centers, as indicated by plaque count, oc- 

 curs for a definite time interval which is characteristic of the virus. 



3. The number of infective centers then increases rapidly for a few 

 minutes until a certain definite number have appeared for each host cell. 



4. The greater the number of virus molecules originally present in the 

 bacterial cell, the more virus molecules are finally formed, but increasing 

 the original number causes much less than the proportional increase in 

 the number formed. Delbriick and Luria assume that the increase in 

 virus observed with increasing infection is due to secondary complica- 

 tions and that actually the number of virus molecules formed per host 

 cell is independent of the infecting number. 



5. Two different viruses, if grown separately, produce the same num- 

 ber of molecules from the same bacterial cell. 



6. If the bacteria are inoculated with two different viruses simul- 

 taneously, only one virus increases. If the bacteria are inoculated with 

 two viruses successively, only the one which is first inoculated increases. 



Delbriick and Luria conclude that the "intracellular" increase in 

 virus is limited by the availability of some substrate, and that liberation 

 takes place when the reaction has run to completion. They assume that 

 a "key enzyme" exists which controls the reaction and which can react 

 with only one virus molecule. This peculiarity accounts for the fact that 

 either virus but not both may increase, and also for the fact that the time 

 for the growth cycle is nearly independent of the number of infective 

 units originally present. It does not account for the fact that the number 

 of molecules formed increases as the number of infective units increases. 



These results may be explained equally well by the precursor mecha- 

 nism, without assuming the existince of a key enzyme. The following 

 assumptions are necessary: 



1, Each host cell contains a definite quantity of precursor. The quan- 

 tity varies depending on the physiological state of the cell. 



2. Virus is formed from this precursor in accordance with the usual 



