426 BACTERIOPHAGES 



techniques involved are much more difficult than those of the 

 neutralization test but may well produce worth-while informa- 

 tion. 



b. Size and Morphology 



Phage particle size as determined by ultrafiltration, centrifu- 

 gation, and X-ray inactivation has been used for purposes of 

 classification. Such methods for size estimation have been 

 superseded by the development of the electron microscope. 

 Size and morphology of phages have been discussed in Chapter 

 IV. It is already evident that there is great variety of size and 

 m_orphology among the bacteriophages and that these character- 

 istics are likely to be more valuable taxonomic criteria for phages 

 than they have been for bacteria. So far as studies have been 

 carried at present there has been complete agreement between 

 the serological and morphological criteria. It is not unlikely 

 that the morphological criterion may be of value at taxonomic 

 levels above the species but no evidence is yet available on this 

 point. 



c. Chemical Composition 



The unique occurrence of hydroxymethylcytosine in place of 

 cytosine in the deoxyribonucleic acid of the related phages T2, 

 T4, and T6 suggests that chemical composition may well be of 

 taxonomic value in phage work. However, it is improbable that 

 chemical analyses will be popular for purposes of classification 

 as long as technically simpler and more direct methods are avail- 

 able. A general survey for the presence of hydroxymethylcyto- 

 sine among a variety of phages may well be enlightening. Only 

 a few phages have so far been analyzed (Chapter VII). 



d. Latent Period 



The latent period is the elapsed time between adsorption of a 

 phage particle to its host cell and lysis of that cell with release of 

 phage progeny. The minimum latent period is a remarkably 



