Schizomycetes, then, are saprophytic or parasitic Schizophyta devoid of chlorophyll, though they may secrete other colouring matters. In size their cells are commonly about 0·001 mm. (called 1 micro-millimetre = lμ) in diameter, or from two to five times that length; but smaller ones and a few larger are known. The various shapes assumed by the cells are shown in fig. 1; the filamentous and other aggregates will be described below.

Schizomycetes are ubiquitous as saprophytes in still ponds and ditches, in running streams and rivers, and in the sea, and especially in drains, bogs, refuse heaps, and in the soil, and wherever organic infusions are allowed to stand for a short time. Any liquid (blood, urine, milk, beer, &c.) containing organic matter, or any solid foodstuff (meat, preserves, vegetables, &c.), allowed to stand exposed to the air soon swarms with bacteria, if moisture is present and the temperature not abnormal. Though they occur all the world over in the air and on the surface of exposed bodies, it is not to be supposed that they are by any means equally distributed, and it is questionable whether the bacteria suspended in the air ever exist in such enormous quantities as was once believed. The evidence to hand shows that on heights and in open country, especially in the north, there may be few or even no Schizomycetes detected in the air, and even in towns their distribution varies greatly; sometimes they appear to exist in minute clouds, as it were, with interspaces devoid of any, but in laboratories and closed spaces where their cultivation has been promoted the air may be considerably laden with them. Of course the distribution of bodies so light and small is easily influenced by movements, rain, wind, changes of temperature, &c. As parasites, certain Schizomycetes inhabit and prey upon the organs of men and animals in varying degrees, and the conditions for their growth and distribution are then very complex. Plants appear to be less subject to their attacks,—possibly, as has been suggested, because the acid fluids of the higher vegetable organisms are less suited for the development of Schizomycetes; nevertheless some are known to be parasitic on plants. Schizomycetes exist in every part of the alimentary canal of animals, except, perhaps, where acid secretions prevail; these are by no means necessarily harmful, though, by destroying the teeth for instance, certain forms may incidentally be the forerunners of damage which they do not directly cause.

Little was known about these extremely minute organisms before 1860. Leeuwenhoek figured Bacteria as far back as the 17th century, and O. F. Müller knew several important forms in 1773, while Ehrenberg in 1830 had advanced to the commencement of a scientific separation and grouping of them, and in 1838 had proposed at least sixteen species, distributing them into four genera. Our modern more accurate though still fragmentary knowledge of the forms of Schizomycetes, however, dates from Cohn’s brilliant researches, the chief results of which were published at various periods between 1853 and 1872; Cohn’s classification of the Bacteria, published in 1872 and extended in 1875, has in fact dominated the study of these organisms almost ever since. He proceeded in the main on the assumption that the forms of Bacteria as met with and described by him are practically constant, at any rate within limits which are not wide: observing that a minute spherical Micrococcus or a rod-like Bacillus regularly produced similar micrococci and bacilli respectively, he based his classification on what may be considered the constancy of forms which he called species and genera. As to the constancy of form, however, Cohn maintained certain reservations which have been ignored by some of his followers. The fact that Schizomycetes produce spores appears to have been discovered by Cohn in 1857, though it was expressed dubiously in 1872; these spores had no doubt been observed previously. In 1876, however, Cohn had seen the spores germinate, and Koch, Brefeld, Pratzmowski, Van Tieghem, De Bary, and others confirmed the discovery in various species.

The supposed constancy of forms in Cohn’s species and genera received a violent shock when Lankester in 1873 pointed out that his Bacterium rubescens (since named Beggiatoa roseo-persicina, Zopf) passes through conditions which would have been described by most observers influenced by the current doctrine as so many separate “species” or even “genera,”—that in fact forms known as Bacterium, Micrococcus, Bacillus, Leptothrix, &c., occur as phases in one life-history. Lister put forth similar ideas about the same time; and Billroth came forward in 1874 with the startling view that the various “form-species” and “formgenera” are only different states of one and the same organism. From that time to the present the discussion as to the limits of “species” among the Schizomycetes has been maintained; much extravagance has resulted, as well as valuable additions to our knowledge of the forms. Klebs (1875) and Nägeli (1877) upheld similar views to those suggested by Lankester; and the researches of Cienkowski, Zopf, Kurth, and De Bary have rendered it clear that forms employed by Cohn to define genera and species (it should be borne in mind that Cohn recognized their provisional nature) occur as phases in one and the same life-history. Zopf showed (1882) that minute spherical “cocci,” short rodlets (“bacteria”), longer rodlets (“bacilli”), and filamentous (“leptothrix”) forms as well as curved and spiral threads (“vibrio,” “spirillum”), &c., occur as vegetative stages in one and the same Schizomycete (cf. fig. 16). In the meantime, while various observers were building up our knowledge of the morphology of the Schizomycetes, others were laying the foundations of what is known of the relations of these organisms to fermenta- tion and disease, —that ancient Will-o’-the-wisp “spontaneous generation” being revived by the way. When Pasteur in 1857 showed that the lactic fermentation depends on the presence of an organism, it was already known from the researches of Schwann (1837) and Helmholtz (1843) that fermentation and putrefaction are intimately connected with the presence of organisms derived from the air, and that the preservation of putrescible substances depends on this principle. In 1862 Pasteur placed it beyond reasonable doubt that the ammoniacal fermentation of urea is due to the action of a minute Schizomycete; in 1864 this was confirmed by Van Tieghem, and in 1874 by Cohn, who named the organism Micrococcus ureae,. Pasteur and Cohn also pointed out that putrefaction is but a special case of fermentation, and before 1872 the doctrines of Pasteur were established with respect to Schizomycetes. Meanwhile two branches of inquiry had arisen, so to speak, from the above. In the first place, the ancient question of “spontaneous generation” received fresh impetus from the difficulty of keeping such minute organisms as bacteria from reaching and developing in organic infusions; and, secondly, the long-suspected analogies between the phenomena of fermentation and those of certain diseases again made themselves felt, as both became better understood. Needham in 1745 had declared that heated infusions of organic matter were not deprived of living beings; Spallanzani (1777) had replied that more careful heating and other precautions prevent the appearance of organisms in the fluids. Various experiments by Schwann, Helmholtz, Schultz, Schroeder, Dusch, and others led to the refutation, step by step, of the belief that the more minute organisms, and particularly bacteria, arose de novo in the special cases quoted. Nevertheless, instances were adduced where the most careful heating of yolk of egg, milk, hay-infusions, &c., had failed,—the boiled infusions, &c., turning putrid and swarming with Schizomycetes after a few hours.

In 1862 Pasteur repeated and extended such experiments, and paved the way for a complete explanation of the anomalies; Cohn in 1872 published confirmatory results; and it became clear that no putrefaction can take place without Schizomycetes. In the hands of Brefeld, Burdon-Sanderson, De Bary, Tyndall, Roberts, Lister, and others, the various links in the chain of evidence grew stronger and stronger, and every case adduced as one of “spontaneous generation” fell to the ground when examined. No case of so-called “spontaneous generation” has withstood rigid investigation; but the discussion contributed to more exact ideas as to the ubiquity, minuteness, and high powers of resistance to physical agents of the spores of Schizomycetes, and led to more exact ideas of antiseptic treatments. Methods were also improved, and the application of some of them to surgery at the hands of Lister, Koch, and others has yielded results of the highest importance.

Long before any clear ideas as to the relations of Schizomycetes to fermentation and disease were possible, various thinkers at different times had suggested that resemblances exist between the phenomena of certain diseases and those of fermentation, and the idea that a virus or contagium might be something of the nature of a minute organism capable of spreading and reproducing itself had been entertained. Such vague notions began to take more definite shape as the ferment theory of Cagniard-Latour (1828), Schwann (1837), and Pasteur made way, especially in the hands of the last-named savant. From about 1870 onwards the “germ theory of disease” has passed into acceptance. Rayer in 1850 and Davaine had observed the bacilli in the blood of animals dead of anthrax (splenic fever), and Pollender discovered them anew in 1855. In 1863, imbued with ideas derived from Pasteur’s researches on fermentation, Davaine reinvestigated the matter, and put forth the opinion that the anthrax bacilli caused the splenic fever; this was proved to result from inoculation. Koch in 1876 published his observations on Davaine’s bacilli, placed beyond doubt their causal relation to splenic fever, discovered the spores and the saprophytic phase in the life-history of the organism, and cleared up important points in the whole question (figs. 10 and 11). In 1870 Pasteur had proved that a disease of silkworms was due to a ferment-organism of the nature of a Schizomycete; and in 1871 Oertel showed that a Micrococcus already known to exist in diphtheria is intimately concerned in producing that disease. In 1872, therefore, Cohn was already justified in grouping together a number of “pathogenous” Schizomycetes. Thus arose the foundations of the modern “germ theory of disease”; and, in the midst of the wildest conjectures and the worst of logic, a nucleus of facts was won, which has since grown, and is growing daily. Septicaemia, tuberculosis, glanders, fowl-cholera, relapsing fever, and a few other diseases are now brought definitely within the range of biology, and several other contagious and infectious diseases are known to be also due to Schizomycetes.

Other questions of the highest importance have arisen from the foregoing. A few years ago Pasteur showed that Bacillus anthracis cultivated in chicken broth, with plenty of oxygen, and at a temperature of 42-43° C. lost its virulence after a few “generations,” and ceased to kill even the mouse; Toussaint and Chaveau confirmed, and others have extended the observations. More remarkable still, animals inoculated with such “attenuated” bacilli proved to be curiously resistent to the deadly effects of subsequent inoculations of the non-attenuated form. In other words, animals vaccinated with the cultivated bacillus showed immunity from disease when reinoculated with the deadly wild form. The questions as to the causes and nature of the changes in the bacillus and in the host, as to the extent of immunity enjoyed by the latter, &c., are now burning,—Metschnikoff’s recent observations (1884), showing that the white corpuscles eliminate the bacilli from the blood, being one of the most startling contributions to the answers.

Another burning question has already been in part touched upon. Experiments have shown that Schizomycetes are pleomorphic; they are also very sensitive, so to speak, to the influences of the environment. The investigations of Cohn, Pasteur, Koch, Nägeli, Kurth, De Bary, and others leave no doubt that many Schizomycetes are sensibly affected by the media in which they are cultivated: not only are the forms modified, but also the physiological activity varies in degree, and even in kind. These and similar facts seem to be largely responsible for recent ideas as to the possibility of being able to cultivate or “educate” certain Schizomycetes. One case only need be referred to. Bacillus anthracis and B. subtilis are only distinguishable with great difficulty morphologically (cf. figs. 10-12); the former is parasitic in its vegetative stages, the latter is always a saprophyte. Now B. anthracis, as said, can become harmless by cultivation, and so it has been thought that the two forms were convertible. Buchner even went so far as to declare that he had transformed B. anthracis into B. subtilis, i.e., that the differences which botanists detect are only due to the influence of the environment at the time. These assertions cannot be regarded as proved; but the question whether harmless forms can become educated, as it were, to a parasitic mode of life within periods which we can control is of course of the highest importance. Such are a few of the questions now under discussion, together with others as to the mode of action of patho- genic Schizomycetes, as to the nature of immunity, and as to the limitation of “species” among such simple forms. Morphology.— Sizes, Forms, Structure, &c.— The Schizomycetes consist of single cells, or of filamentous or other groups of cells, according as the divisions are completed at once or not. While some unicellular forms are less than 1 μ (·001 mm.) in diameter, others have cells measuring 4μ, or 5 μ or even 7 μ or 8 μ in thickness, while the length may vary from that of the diameter to many times that measurement. In the filamentous forms the individual cells are often difficult to observe until reagents are applied (e.g., fig. 14), and the length of the rows of cylindrical cells may be many hundred times greater than the breadth. Similarly, the diameters of flat or spheroidal colonies may vary from a few times to many hundred times that of the individual cells, the divisions of which have produced the colony. The shape of the individual cell (fig. 1) varies from that of a minute sphere to that of a straight, curved, or twisted filament or cylinder, which is not necessarily of the same diameter throughout, and may have flattened, rounded, or even pointed ends. The rule is that the cells divide in one direction only— i.e., transverse to the long axis—and therefore produce aggregates of long cylindrical shape; but in rarer cases iso-diametric cells divide in two or three directions, producing flat, or spheroidal, or irregular colonies, the size of which is practically unlimited. As to the structure of the cell, little more can be said than that it consists of a mass of homogeneous or very slightly granular protoplasm, with a pearl-like lustre, and without vacuoles; this is enveloped by a membranous envelope, which is so delicate as to be scarcely perceptible. In the actively vegetating or mobile conditions this cell wall appears very thin and sharp, and is extremely flexible and elastic, but at other times it is swollen and diffluent, furnishing the intercellular gelatinous matrix of the zoogloea condition (fig. 3). It is doubtful whether the thin envelope closely applied to the protoplasm is not always simply the innermost layer of a very diffluent covering, which is continuously thickening and throwing off its outermost swollen and disorganized lamellae. The facts to hand seem to show that, while in some cases this envelope consists mainly of cellulose, in others (zoogloea of Bacteria, e.g.) it contains relatively large proportions of nitrogenous compounds. In some cases the cell-walls form a lamellated sheath. No cuticularization occurs, nor are deposits of lime or silex known in the cell walls. Colouring pigments, however (red, yellow, and even green and blue), are sometimes met with, and a rusty or brown tinge is in some cases produced by the precipitation of iron oxides in the walls. In the typical Schizomycetes the protoplasmic contents (which are said to consist largely of a peculiar substance named mycoprotein) are colourless, or more rarely tinged with colouring matters—bright red, yellow, &c.—which cannot be mistaken for chlorophyll. The few forms described as containing a green pigment, allied to or identical with chlorophyll, will not be considered here, but relegated to the Algse. The occurrence of starch or a granulose-like substance in some Bacteria is undoubted; it yields a deep blue colour with iodine solutions, is diffused in bands or patches, and arises in cases where the Schizomycete is nourished by a matrix which does not contain starch. Trécul noticed this formation of amyloid substance in Clostridium, Van Tieghem in a Spirillum, and several other cases are known; Ward detected starch in a Bacillus found in decaying coffee seeds, and in other media devoid of starch. In the filamentous Schizomycetes (Beggiatoa, e.g.) are found extremely minute dark granules; Cramer and Cohn have shown that these consist of sulphur in fine crystals (fig. 14). Oily or fatty substances and minute granules of undetermined nature occur in the protoplasm, but no nucleus has as yet been discovered in any Schizomycete.

Vegetative States.— While many forms are fixed to a substratum, others are free; and in certain conditions single cells or groups may be motile. In some cases the movements are mere oscillations, in others there are rapid movements of translation, sometimes ascribed to the action of flagella or cilia; these movements are of course not to be confounded with the dancing “Brownian motion” observed in the case of all such minute bodies suspended in fluids. Cilia have now been described in some of the smallest Bacteria by several good observers (Dallinger and Drysdale, Cohn, Koch, Zopf), though, on account of their extreme fineness, and the difficulty of fixing them, much discussion has taken place as to their nature, functions, origin, numbers, and even existence; that they occur is proved by the photographs, but whether they are not sometimes mere filaments drawn out from the cell-walls is very doubtful

(figs. 2 and 12).

While some Schizomycetes appear to have no active stage, and many are only motile under certain conditions when swarming, others are described as possessing two or even three distinct active forms.

When vigorously growing and dividing, the Schizomycetes as a rule present certain definite forms, which are at any rate so constant under constant conditions that they can be figured and described with such accuracy and certainty that good observers have regarded them as fixed species, or at least as “form-species” or “form-genera.” We now know, however, that many Schizomycetes pass through several such phases, and we may therefore regard them in these cases as “vegetative forms,” which pass into one another too gradually to admit of their being employed as sharply distinctive of genera.

As the chief of these forms may be mentioned the following (see fig. 1) :— Cocci : spherical or spheroidal cells, which, according to their relative (not very well defined) sizes are spoken of as Micrococci, Macrococci, and perhaps Monas forms.

Rods or rodlets∙. slightly or more considerably elongated cells which are cylindrical, biscuit-shaped, or somewhat fusiform. The cylindrical forms are short, i.e., only three or four times as long as broad (Bacterium), or longer (Bacillus) ; the biscuit-shaped ones are Bacteria in the early stages of division. Clostridia, &c., are spindle-shaped.

Filaments (Leptothrix forms) really consist of elongated cylindrical cells which remain united end to end after division, and they may break up later into elements such as those described above. Such filaments are not always of the same diameter throughout, and their segmentation varies considerably. They may be free, or attached at one (the “basal ”) end. A distinction is made between simple filaments (e.g., Leptothrix) and such as exhibit a false branching (e.g., Cladothrix). Curved and spiral forms. Any of the elongated forms described above may be curved, or sinuous, or twisted into a corkscrew-like spiral instead of straight. If...