EXPLANATION OF PLATE II. 



This cut shows the appearances which are presented, %fter the 

 germs have grown, by particles of sand and shreds of vegetable fibre 

 to which single germs were clinging when they settled on to the un- 

 covered gelatin plate. In this case the drawing was made five days 

 after the exposure of the plate to the air of a dusty street. The largest 

 of these colonies were barely visible to the naked eye. 



I. Shows a particle of sand completely surrounded by the colony 

 or mass of bacteria which has grown from a single germ which was 

 clinging to the minute sand particle as it settled with the dust. 



2. Shows a tiny shred of wood to which five different germs were 

 attached as it settled on to the exposed plate. We should probably 

 have searched in vain, even with a powerful microscope, for the single 

 germs clinging to it at the time this wooden dust particle planted itself 

 on the surface of the gelatin. But now the larger colonies are visible 

 even to the naked eye. We know that they grew from different 

 species of germs because under a moderate magnifying power they 

 present such markedly different appearances. 



3. Shows a minute sliver to which four different forms of germs 

 were clinging as it fell. 



