SIZE AND MORPHOLOGY OF BACTERIOPHAGES 41 



served earlier in unpurified lysates by Herriott and Barlow 

 (1952). Avery similar structure for the tail of the staphylococ- 

 cus phage K had been surmised by Hotchin (1954). The func- 

 tional significance of this structure is discussed by Williams and 

 Fraser (1956). 



Discontinuity of internal structure of the phage head is notice- 

 able in certain types of preparation (Luria, Delbruck, and 

 Anderson, 1943; Terada, 1956). Unlike the tail structure de- 

 scribed above, which undoubtedly reflects functional differentia- 

 tion, the appearance of the head contents may depend on acci- 

 dents of drying (T. F. Anderson, Rappaport, and Muscatine, 

 1953). 



2. Ultrafiltration 



Some of the earliest attempts to measure the size of phage 

 particles sought to determine the largest pore diameter of col- 

 lodion membranes through which the particles could not pass. 

 The method was chiefly developed by Elford, whose review (El- 

 ford, 1938) is the source of the information given here. The 

 method is based on the following principles. 



A series of graded collodion membranes can be prepared by 

 systematically varying the solvent mixture from which the mem- 

 brane is precipitated. Measurements of the rate of flow of water 

 through such membranes yield, with the aid of hydrodynamic 

 theory, a nominal average pore diameter. Finally, an empirical 

 factor, arrived at by testing substances of known particle size, 

 relates the average pore diameter of the membrane to the size of 

 particles it will hold back. This factor presumably corrects for 

 heterogeneity of pore size, adsorption, electrostatic effects, and 

 other imponderables. (Adsorption is minimized by suspending 

 the phage in peptone solutions.) Elford multiplied the hydro- 

 dynamic pore diameter of the coarsest membrane that would 

 fail to pass phage by factors varying between 0.33 and 0.50, de- 

 pending on the membrane, to obtain the diameter of the phage 

 particle. The sizes he determined have proved to be somewhat 

 too small. Approximately correct diameters are obtained from 



