12 MORPHOLOGY 



plast is invested bv a membrane, appears to be homogeneous, and usu- 

 ally contains a few granules. It seems not to have even such simple 

 differentiation as occurs in the cells of certain Cyanophyceae ; but in 

 all such cases it must be kept in mind that we are dealing with very 

 minute objects and that presently a technique may be developed that 

 will reveal an amount of organization that we have no means of seeing 

 at present. 



Many of the bacteria are ciliated, the cilia being distributed over the 

 body in various ways, and always extremely difficult to detect. These 

 ciliated forms are very active, and their movements have suggested 

 that bacteria are animals. Under certain conditions many bacteria 

 pass into a quiescent stage and collect in colonies that are held together 

 by a gelatinous matrix formed from the walls. These quiescent colonies, 

 thus embedded, form characteristic pellicles on nutrient media, as on a 

 decoction of hay, on bouillon, on stagnant water, and on various solid 

 media. This quiescent, pellicle-forming stage is known as the zoogloea 

 stage. 



Multiplication. The multiplication of cells by division is exceedingly 

 rapid, the progeny of one cell in twenty-four hours often running into 

 many millions. As already said, these newly formed cells either sepa- 

 rate or hang together in filaments. When the nutritive supply fails, 

 the protoplasm condenses in the middle or end of the cell and becomes 

 invested by a heavy membrane. These are the so-called spores, but 

 they are really resting cells such as are formed among the Cyanophyceae, 

 except that they are formed within the old cell. These resting cells are 

 even more resistant than the ordinary vegetative cells. In favorable 

 conditions the protecting membrane bursts and the protoplast resumes 

 active division. 



Cultures. It is very difficult and often impossible to recognize species 

 of bacteria by the appearance of the individual cells, but in mass cultures 

 the colonies are often very distinct in* form, color, structure, and effect 

 on nutrient media. These mass cultures are made in liquid media or 

 upon solid media (gelatin, agar, potato, etc.). For purposes of inves- 

 tigation pure cultures are absolutely necessary, which means the sepa- 

 ration of the form under investigation from every other form with which 

 it may be associated, a process requiring a special technique. 



Activities. Many bacteria are peculiar in that they are able to live 

 in the absence of free oxygen, which in other plants is associated 

 with the fundamental process of respiration. Such bacteria are called 



