Head 



Contracted 

 sheath 



r 



ore 



ft Plate 



I art 



fibre 



Fig. 3. Virus structure — Bacteriophage. The Bacteriophage has a 

 more complex structure than a simple virus such as the tobacco 

 mosaic virus. It has a well-developed head and a tail. The head 

 contains the nucleic-acid component and this flows through the 

 tail into the bacterium it attacks and there reproduces. (This 

 photograph was obtained through the kindness of Dr. S. Brenner 



and Dr. R. W. Home.) 



