Under favorable conditions bacteria multiply rapidly, but when conditions are unfavorable, they take on a spore formation and remain in a quiescent state, like the seed of a plant, waiting—it may be years—until proper conditions are present. The spores are protected by such a thick envelope and have such great potential vitality, that it is much more difficult to kill them than the developed bacteria. Certain spores that withstand 212°F. or 100°C., can be killed when fully developed at 130°F. or 55°C. +Toxins.+ As bacteria grow, certain poisonous chemical substances appear about them. These poisons are produced by them directly, or are formed in the organic matter or tissues in which they live, as the result of their presence. Some of these substances are alkaloidal and are known as _ferments_ or _ptomains_. Others are albuminous in nature and are called _toxalbumins_. The ptomains and toxalbumins are exceedingly powerful poisons, producing local necrosis, inflammation and even suppuration, when introduced by themselves and entirely free from living germs, into the tissues of animals. Pathogenic bacteria abstract the lymph from the blood. As the lymph contains elements necessary to the body, such as water, oxygen, albumins, carbohydrates, etc., their loss brings about body-waste and exhaustion from lack of nourishment. Again, bacteria produce a vast number of compounds, some harmless and others highly poisonous. The symptoms of a microbic disease are largely due to the absorption of poisonous materials from the area of infection. These poisons may be formed in the tissues by the action upon them of the bacteria, or they may be liberated from the bodies of degenerating microbes. Bacteria secrete and contain ferments like pepsin or trypsin, and as albumoses are formed in the alimentary canal by the action of the digestive ferments upon proteids, sugars, and starches, so microbic albumoses are formed by the action of microbic ferments upon tissues. The local and general symptoms of these toxins depend upon the particular toxin employed and a large number of these poisons have been isolated and studied. Those of the surgically important pathogenic germs, produce inflammation locally, with general symptoms of fever, chills, cardiac depression, irritation of the kidneys and bowels and cerebral symptoms, such as delirium and coma. The toxalbumins also appear to have the effect of destroying the bacteria to which they owe their origin when they have been produced in large quantity. +Cultivation.+ Bacteria are cultivated for study in the laboratory in meat extracts, in gelatine, or agar agar (a sort of vegetable gelatine), or raw potato, in blood serum and in other materials. The simplest method of cultivation is in bouillon, sterilized in flasks, with cotton plugs. Another method of studying bacteria is by the inoculation of animals.