which they take up colouring matter-such granules are to be seen in the diphtheria bacillus.

By a delicate process of extraction and subsequent staining with hematoxylin the central plasm of the organism has been shown by Bütschli to be finely reticulated, and that the plasm appears differentiated into a central portion and a parietal layer besides the cell wall; the central mass is therefore a large nucleus, the parietal layer corresponding to the cytoplasm of higher plants.

1, Types of spore-forming bacilli; 2, three types of spore germination; 3, flagellæ: (a) peritrichic, (b) lophotrichic, (c) monotrichic.

Most bacteria are surrounded with a gelatinous, homogeneous covering termed capsule; in some species-as the pneumococcus of Fränkel and the pneumobacilli of Friedländer-it may be easily demonstrated. It is not always present in cultivations, and is seen best in preparations made from the blood of an animal which has succumbed to infection with the organism. The capsule is also seen in specimens obtained from pneumonic lung and "prune juice" sputum.

Many organisms possess the power of independent movement and are described as motile. Upon such organisms extremely

slender whip-like filaments may be observed by special methods of staining, known as flagella, and may be single and situated at one end, or the organism may be thickly studded with them; they are often many times the length of the organism. Owing to their minute size it is difficult to say to which part of the bacterial cell they are primarily attached. With the exception of a few doubtful species motility is confined to the bacilli and spirilla. By examination of hanging drop preparations the motion can be easily observed, and the organisms seen darting about in all directions. The absence of motility is not always synonymous with absence of flagella, many physical conditions causing "torpidity."

Bacteria possessing flagella are classed as (a) Peritrichic, multiple flagella surrounding the bacillus; (b) Lophotrichic, tufts of polar flagella; (c) Monotrichic, single polar flagella. (Fig. 2.)

FIG. 3.-INVOLUTION FORMS OF BACTERIA.

Another form of movement which must not be confounded with the above is what is known as "Brownian movement," consisting of a dancing, oscillating swing of the organisms. It is a purely physical condition occurring with solutions of inorganic matter, exactly as with non-motile organisms, and is probably related to surface tension. In determining motility, observe the position of three organisms situated at the angles of an imaginary triangle and watch for a change of relative position, carefully excluding mechanical shock.

Spore Formation.-Various organisms of the class bacilli produce within their plasm highly refractile bodies which are capable of withstanding higher temperatures and stronger disinfectant solutions than the vegetative form; these bodies are spores.

The spores take up staining reagents with great difficulty, special treatment being necessary (see chapter on microscopical methods). Spore formation consists of a condensation of the cell

plasm, the spore thus formed being surrounded with a tough membrane consisting of two layers. At the same time the remainder of the organism undergoes degenerative changes, eventually setting free the mature spore. Certain other clear spaces exist from time to time in the bacterial plasm, filled with lustrous drops of a fatty nature rendering observation alone useless in the determination of sporulation (page 47).

The number of spores formed by a bacillus is rarely more than one, and is sometimes situated at one end, when the "drum stick " form is produced (Tetanus bacillus, fig. 2, 1).

In some species (B. alvei), when the spore is centrally situated the cell plasm alters in form and becomes massed around the spore, forming a clostridium or spindle-shaped mass.

The form and size of the spores differ greatly in various species but is constant for the same species. B. subtilis has ellipsoidal spores 1.2 length by about 0-6μ breadth.

μ

These spores formed in the interior of bacteria, or endospores as they are termed, are highly resistant to the action of heat, disinfectants, or light, and a proper knowledge of them is of supreme importance in all bacteriological and hygienic work.

Gruber and Brefield found that the spores of B. subtilis required three hours' boiling at 100° C. to kill them, whereas the rod-shaped forms were easily destroyed by heating for twenty minutes at the temperature of boiling water. When the spore is exposed to favourable conditions it germinates, and in a short time (three to four hours for B. subtilis, Prazmouski) produces the parent form. The process is of three types:

(1) The spore gradually elongates, the outer membrane disappearing, eventually reaching the adult form which divides by binary fission, in the resulting cells spore formation again taking place.

(2) The spore membrane is ruptured at the point of least resistance, and gradually grows out from the empty spore capsule, which dissolves in the surrounding fluid. The capsule, especially in motile forms, may be seen attached to the free-swimming rod.

(3) The spore membrane ruptures at the equator of the spore, the developing rod gradually forcing its way out by one pole; a portion of the spore may remain attached.

Plasmolysis, or shrinking of the cell plasm with resulting spaces between the plasm and cell wall, may be produced by the action of

certain reagents, among them being the ordinary physiological salt solution (0.75 gm. NaCl in 100 gms. of water). Various staining reagents bring about this phenomenon. The plasmolytic effect may be produced and removed without any apparent injury to the living cell.

CHAPTER II.

The Biology of Bacteria.

BACTERIA are greatly influenced by their surroundings, and on the other hand often profoundly modify the substratum in which they are growing, be it living tissue or nutrient solution, whilst the very products resulting from their activity are among the chief inhibitory influences restraining their indefinite development.

Cohn estimated that a single bacillus 2 μ long and 1 μ broad, weighing 0.000000001571 mgrm., and which reproduced itself by binary fission once in half an hour, will in two days' time have a progeny of 281 billions, occupying a volume of half a litre, while in another three days the mass would be sufficient to fill the beds of all the oceans of the globe, the number of the progeny being represented by 37 places of figures! That such enormous development does not take place is due partly to the antagonism displayed by one species towards another, partly by the insufficiency of nutriment obtainable, but chiefly to the products of the organisms' own activity. The more important conditions related to the development of bacteria are-light, temperature, gaseous environment, moisture, food supply. The first four factors are related more particularly to the development of the bacteria, whilst the question of food supply is largely complicated by the various chemical changes induced by bacterial action.

Effect of Light.-The antagonism of light to disease was a fact established by empirical observation long before bacteria were known to exist, and the old Italian proverb, "Where the sun does not enter, the doctor does," is illustrative of this popular knowledge gained by observation. Since the discovery of bacteria many experiments have proved the scientific basis of the empirical deduction; the majority of bacteria, and certainly all the known pathogenic forms, are particularly sensitive to the action of light.

Direct sunlight is the most powerful agent; exposure to the