From the medium. Unwashed preparations contain proteins, sugars, and cell debris (below). They yield dirty, poorly contrasted, even opaque images with little structural details (Fig. S4) . PEG (polyethylene glycol), which is frequently used to concentrate phages, produces similar effects. PEG will collect anything: DNA, proteins, cell debris, phages and phage debris. Fortunately, PEG is is easily removed by washing. In one recent case, four washes were needed to obtain a clean preparation. CsCl, used for phage purification, may persist after incomplete dialysis. CsCl forms flat crystals, but does not interfere with staining. Arborescent or flat NaCl crystals are frequent in lysates from halophilic bacteria which require NaCl for growth, e.g., Vibrio and Halobacterium. Again, bacteriophages may act as nuclei of crystallization (Fig. S18) . NaCl crystals may obscure phage particles, but do not alter the quality of staining. In lysates from bacteria prepared with meatbased media, e.g., of clostridia, one may observe occasional fibers of striated muscle and bundles of double-walled rings which resemble phage tails (Fig. S41) and may be be attributed to self-assembly under the action of phospholipase C. 3 From bacteria (Table 2 ). Bacteria contribute proteins, DNA, ribosomes, cytoplasm, capsular material, pili, flagella, and fragments of plasma membrane and cell wall. Proteins and cytoplasm may interfere with the staining process, but are not a source of error. However, this is the case with the other cellular constituents that one may find in a lysate. Their presence depends very much on the phage host. For example, slime and capsular material are present in huge amounts in lysates of Acinetobacter and Xanthomonas and cell wall debris abound in those of Pseudomonas.