oval bodies situated at the ends of the cell. Their significance is unknown. They have been supposed to bear some relationship to the biologic activity of the organism, especially its pathogenesis, but this is uncertain, and a recent work by Gauss* and Schumburg shows that they vary with the reaction of the culture-media upon which the bacteria grow and have nothing to do with their virulence. Bacillus megatherium is peculiar in having its cytoplasm filled with small granules which stain deeply. The diphtheria bacillus and the cholera spirillum stain very irregularly in fresh cultures, as if the tingeable substance were not uniformly distributed throughout the cytoplasm. Vacuolated bacteria and bacteria that will not stain, or stain very irregularly, may usually be regarded as degenerated organisms (involution forms) which, because of plasmolysis, or solution, can no longer stain homogeneously.

Flagella. Many bacteria possess delicate straight or wavy filaments, called flagella, which appear to be organs of loco

motion.

Messea has suggested that the bacteria be classified, according to the arrangement of the flagella, into:

I. Gymnobacteria (forms without flagella).

II. Trichobacteria (forms with flagella).

1. Monotricha (with a single flagellum at one end).

2. Lophotrocha (with a bundle of flagella at one end).

3. Amphitricha (with a flagellum at each end).

4. Peritricha (flagella around the body, springing from all parts of its surface).

This arrangement is, however, less satisfactory than that of Migula already given.

Motility. The greater number of the bacteria supplied with flagella are actively motile, the locomotory power no doubt being the lashing flagella. The rod and spiral microorganisms are most plentifully supplied with flagella; only a few of the spheric forms have them.

The presence of flagella, however, does not invariably imply motility, as they may also serve to stimulate the passage of currents of nutrient fluid past the organism, and so favor its nutrition. The flagellate bacteria are more numerous among the saprophytic than the pathogenic forms.

* "Centralbl. f. Bakt.," etc., xxxI, No. 3, Feb. 5, 1902, p. 106.

† Ibid., XXXI, No. 14, p. 694, June 3, 1902.

"Rivista d'igiene e sanata publica," 1890, II.

Bacillus megatherium has a distinct but limited ameboid movement.

The dancing movement of some of the spheric bacteria seems to be the well-known Brownian movement, which is a physical phenomenon. It is sometimes difficult to determine whether an organism viewed under the microscope is really motile or whether it is only vibrating. One can usually determine by observing that in the latter case it does not change its relative position to surrounding objects.

In some cases the colonies of actively motile bacteria, such as the proteus bacilli, show definite migratory tendencies upon 5 per cent. gelatin. The active movement of the bacteria composing the colony causes its shape constantly to change, so that it bears a faint resemblance to an ameba, and moves about from place to place upon the surface of the gelatin.

Size.-Bacteria are so minute that a special unit has been adopted for their measurement. This is the micromillimeter (u), or one-thousandth part of a millimeter, equivalent to the one-twenty-five-thousandth (250) of an inch.

The size of bacteria varies from a fraction of a micromillimeter to 20 or even 40 micromillimeters.

Reproduction.-Fission.-Bacteria multiply by binary division (fission). A bacterium about to divide appears larger than normal, and, if a spheric organism, more or less ovoid. By appropriate staining karyokinetic changes may be observed in the nuclei. When the conditions of nutrition are good, fission progresses with astonishing rapidity. Buchner and others have determined the length of a generation to be from fifteen to forty minutes.

The results of binary division, if rapidly repeated, are almost appalling. "Cohn calculated that a single germ could produce by simple fission two of its kind in an hour; in the second hour these would be multiplied to four; and in three days they would, if their surroundings were ideally favorable, form a mass which can scarcely be reckoned in numbers." "Fortunately for us," says Woodhead, "they can seldom get food enough to carry on this appalling rate of development, and a great number die both for want of food and because of the presence of other conditions unfavorable to their existence."

Sporulation. When the conditions for rapid multiplication by fission are no longer good, many of the organisms guard against extinction by the formation of spores (Fig. 1).

Endospores, or spores developed within the cells, are generally formed in the elongate bacteria,-bacillus and spirillum, but Zopf has observed similar bodies in micrococci. Escherich also claims to have found undoubted spores in a sarcina.

Spores may be either round or oval. As a rule, each

Fig. 1.-Diagram illustrating sporulation: a, Bacillus inclosing a small oval spore; b, drumstick bacillus, with the spore at the end; c, clostridium; d, free spores; e and ƒ, bacilli escaping from spores.

organism produces a single spore, which is situated either at its center or at its end. When, as sometimes happens, the diameter of the spore is greater than that of the bacillus, it causes a peculiar barrel shape bulging of the organism, described as clostridium. When the distending spore is at the end, a "Trommelschläger," or "drumstick," is formed. End-spores are almost characteristic of anaerobic bacilli. When the formation of a spore is about to commence, a small bright point appears in the cytoplasm, and increases in size until its diameter is nearly or quite as great as that of the bacterium. A dark, highly refracting capsule is finally formed about it. As soon as the spore arrives at perfection the bacterium seems to die, as if its vitality were exhausted.

The spores differ from the bacteria in that their capsules prevent evaporation and enable them to withstand drying and the application of a considerable degree of heat. Very few adult bacteria are able to resist temperatures above 70° C. Spores are, however, uninjured by such temperatures, and can even successfully resist the temperature of boiling water (100° C.) for a short time. The extreme desiccation caused by a protracted exposure to a dry temperature of 150° C. will invariably destroy them, as will also steam under pressure. Not only can the spores successfully resist a considerable degree of heat, but they are also unaffected by cold of almost any intensity. Von Szekely* found anthrax spores capable of germination after eighteen years

* "Zeitschr. für Hygiene," 1903, XLIV, 3.

and six months in some dried-up old gelatin cultures found in his laboratory.

Arthrospores.-The formation of arthrospores is less clear, and seems to be the conversion of the entire organism into a spore or permanent form. Arthrospores have been observed particularly among the micrococci, where certain individuals become enlarged beyond the normal, and surrounded by a capsule.

Though the cell-wall of the adult bacterium is easily penetrated by solutions of the anilin dyes, it is difficult to stain spores, which are distinctly more resistant to the action of chemic agents than the bacteria themselves.

Germination of Spores.-When a spore is about to germinate, the contents, which have been clear and transparent, become granular, the body increases slightly in size, the capsule becomes less distinct, and in the course of time splits open to allow the escape of a young organism. The direction in which the capsule ruptures varies in different species. Bacillus subtilis escapes from the side of the spore; Bacillus anthracis from the end. This difference can be made use of as an aid in differentiating otherwise similar organisms.

So soon as the young bacillus escapes it begins to increase in size, develops a characteristic capsule, and presently begins the propagation of its species by fission.

Morphology. The three principal forms of bacteria are spheres (cocci), rods (bacilli), and screws (spirilla).

Cocci.-The spheric bacteria, from a fancied resemblance to little berries, are called cocci or micrococci. When they divide, and the resulting organisms remain attached to one another, a diplococcus is produced. Diplococci may consist of two attached spheres, though each half commonly shows flattening of the contiguous surfaces. In a few cases, as the gonococcus, the approximated surfaces may be slightly concave, causing the organism to resemble the German biscuit called a "Semmel." When a second binary division occurs, and four resulting individuals remain attached to one another, without disturbing the arrangement of the first two, a tetrad, or tetracoccus, is formed. To the entire groups of cocci dividing in two directions of space so as to produce fours, eights, twelves, etc., on the same plane, the name merismopedia has been given. Migula uses the term micrococcus for the unflagellated tetrads, and planococcus for the flagellated forms.

If division take place in three directions of space, so as to produce a cubic "package" of cocci, the resulting aggregation is described as a sarcina. This form resembles a dice or a miniature bale of cotton. Few sarcinæ have flagella, similar flagellated organisms being called by Migula planosarcina.

If division always take place in the same direction, so that the cocci remain attached to one another like a string of beads, the organism is described as a streptococcus.

Cocci commonly occur in irregular groups having a fancied resemblance to bunches of grapes. Such are called

Fig. 2.-Diagram illustrating the morphology of the cocci: a, Coccus or micrococcus; b, diplococcus; c, d, streptococci; e, f, tetracocci or merismopedia; g, h, modes of division of cocci; i, sarcina; j, coccus with flagella; k, staphylococci.

staphylococci, and most organisms not finding a place in the varieties already described are so classed.

Cocci associated in globular or lobulated clusters incased in a resisting gelatinous, homogeneous mass, have been described by Billroth as ascococcus, Cocci, solitary or in chains, surrounded by an incasement of almost cartilaginous consistence, have been called leuconostoc.

Bacilli.-Better known, if not more important, bacteria consist of elongate or "rod-shaped forms," and bear the name bacillus (a rod). These present considerable variation of form. Some are ellipsoid, some long and slender. Some have rounded ends, as Bacillus subtilis; others have square ends, as B. anthracis. Some are large, some exceedingly small. Some always occur singly, never uniting to form threads or chains; others are nearly always so conjoined.

The bacilli divide by transverse fission only, so that the only peculiarity of arrangement is the formation of threads or chains. In the older writings, short, stout bacilli were