162 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



and in the depth of the plate. Those on the surface are usually 

 flat with raised centres, and of varying shades of crimson. 

 The central part is usually deeper in colour than the margin. 

 Sometimes they have very little colour, but one can always 

 distinguish them from the transparent colourless colonies. 

 The latter are generally slightly irridescent, whereas the others 

 (the crimson colonies) are slightly opaque even if they have 

 very little colour. The colonies in the depths are always 

 much smaller, and their colour is much deeper. The larger 

 ones are egg-shaped when seen from the surface, and there 

 is generally a slight haze round them, easily seen if the medium 

 is clear. In cultures from mussels, made as I have described, 

 we inoculate a complex physical mixture, a suspension, largely 

 of broken-down solid tissue, and this often prevents the haze 

 round the deep colonies from being visible. These deep 

 colonies are really lenticular in shape, with their long diameters 

 perpendicular to the surface of the plate. They assume this 

 position, I suppose, because they grow more easily towards 

 the surface than parallel to it. 



Now these differences in the naked-eye appearance of 

 the red colonies are of little or no significance : we cannot 

 tell, except by subculturing a colony, what micro-organism 

 it represents. The differences are produced by the varying 

 positions of the colonies in the medium, and by the degree 

 of aggregation of the colonies. Consider what is the physical 

 structure of an agar plate. It is an emulsoid consisting of 

 two liquid phases. In a dilute solution of agar, like that 

 employed in the preparation of " solid " nutritive media, 

 cooling throws out of true solution a phase consisting of 

 droplets of a relatively concentrated solution of agar in water 

 (or water in agar). These droplets coalesce together to form 

 a meshwork in the interstices of which is the second phase, 

 the relatively dilute solution of agar in water. The liquids 

 of the two phases do not separate into layers, but remain 



