104 BENZER ET AL, 



Slender tail 150 rnji long, not more than 15 m[i thick. Tail appears 

 straight or slightly curved (Wyckoff, 1949; Luria and Williams, per- 

 sonal communication). 



T2: Large head 65 x 80 m[^i, oval or in the form of a short rod 

 with conical or rounded ends. Slender straight tail 120 m|.i long and 

 20 m^x thick, terminating in a slightly broader part. There seems to be 

 an internal structure in the head (Hook et al., 1946). 



T4, T6 are indistinguishable from T2. All three show "ghosts" 

 in older suspensions, and in preparations washed in distilled water 

 (osmotic rupture), and in preparations treated with NaOH, or with 

 sonic vibrations, or heavily irradiated with UV. These ghosts seem to 

 be empty membranes (see below: Osmotic shock) (Anderson, 1949). 



T5: Round head of 100 m[i diameter, distinct tail (Anderson, 

 1946). 



T3: Round head of 45 mu diameter, no tail (Anderson, 1946). 



T7: Round (but not smooth) head of 51 mfi. The heads seem to 

 contain a bilobular structure. Except for this, the particles seem essen- 

 tially homogeneous without peripheral material. No tail (Kerby et al., 

 1949). 



It has been claimed that T2 grown in synthetic mediimi is larger 

 than T2 grown in broth (about 10%) (Sharp et al., 1946), that T3 

 has a diameter of 35 mfi, that both T3 and T7 show very slender tails 

 in some preparations (Wyckoff, 1949), and that the diameter of T7 is 

 73 m[x when chromium shadowed (Sharp et al., 1949). 



One must not forget that all the preparations seen in the electron 

 microscope have been radically dried. 



Indirect methods (sedimentation in centrifuge, diffusion, etc.) have 

 given values for the dimensions of the phages compatible with those 

 of the electron microscope. 



(b) Osmotic Shock. 



T2, T4, T6, which appear in the electron micrographs to have 

 membranes, can be disintegrated by "osmotic shock." "Osmotic shock" 

 is produced by incubation of the viruses in 4M NaCl, followed by a 

 rapid dilution of the suspension in distilled water. The viruses are in- 

 activated by this process, and electron micrographs of the suspensions 

 show "ghosts," i.e., empty head membranes with tails still attached. 

 Ti, T3, T7 show no membrane in the electron microscope and are not 



