M. DELBRUCK 649 



teria were infected with about an equal number of phage or with a large 

 excess of phage (200 to 1 or still higher ratios) and these will be described 

 separately. 



1. PIB = 1 



(a) Hanging Drop 



No changes in the shape of the bacteria were observed, only a gradual 

 diminution in the number of bacteria. 



iJS) On Nutrient Agar Plates 



The bacteria were first mixed with phage in broth and aerated at 37° 

 for 10 minutes, as in an ordinary growth curve set up. At 10 minutes a 

 0.1 cc. sample was spread on an agar plate and observed under the micro- 

 scope (magnification 600). A map was drawn of about 100 bacteria in the 

 field of vision and these were checked every 5 minutes for changes. Up to 

 about 30 minutes (now at room temperature, 25°) only a few bacteria dis- 

 appeared, the others showed no change. Between 30 and 70 minutes 70 

 per cent of the bacteria disappeared. They disappeared by a process of 

 fading out, without noticeable change in shape. The fading out takes about 

 2 minutes, a faint outline of the rod remaining visible for a long time after- 

 wards. There appeared to be no correlation between the size of the bac- 

 terium and the inception of its fading. Many bacteria which at the be- 

 ginning had a constriction as if they were on the point of dividing behaved 

 as one bacterium on lysis. Sometimes the fading appeared to start at one 

 end of the rod and then to proceed gradually through its entire length. 



Discussion. — The bacteria are visible without staining under the micro- 

 scope by virtue of a difference in the refractive index between their interior 

 and the surrounding medium. The fading out without change of shape 

 means then that the refractive index of its contents becomes equal to that 

 of its milieu, while the cell wall retains its form. Inside and outside must 

 suddenly become capable of free exchange. This must be due to a sudden 

 disruption of the protoplasmic membrane. A change in permeability could 

 hardly be so drastic as to permit complete equalization in so short a time. 



2. P/B = 200 or greater 

 {a) Hanging Drop 



The bacteria kept their normal size and rod shape up to about 20 minutes. 

 Then suddenly within 1 or 2 minutes the large majority was transformed 

 into spherical bodies of about the same volume and small refractive power. 

 These spherical bodies were visible for a long time and only gradually de- 

 creased in number, some could be seen much distended and of oval shape. 



Besides these spherical bodies there appeared a few very minute rods that 

 were extremely motile. 



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