SCHIZOMYCETES, a term proposed by Niigeli in 1857 to include all those minute organisms known as Bacteria, Microphytes, Microbes, &c., and allied forms. These terms have been used at various times by different authors with widely different meanings in detail, but it is now agreed that the Schizomycetes are minute vegetable organisms devoid of chlorophyll and multiplying by repeated bipartitions. They consist of single cells, which may be spherical, oblong, or cylindrical in shape, or of filamentous or other aggregates of such cells. True spores occur in several, but no trace whatever of sexual organs exists. From their mode of growth, division, and spore-formation (in part), as well as their habit of forming deliquescent, swollen cell-walls, and other peculiarities, there can be no doubt of the close alliance between the Schizomycetes and certain lower Alga;; whence both groups have been conjoined under the name Schizophyta. No one character except the want of chlorophyll - which of course entails physiological differences - separates the Schizomycetes from other Schizophyta; morphologically and phylogenetically the two groups are united. From this point of view we relegate all the so-called bacteria which contain chlorophyll (e.g., Engelmann's Bacterium chlorinum, Van Tieghem's B. uiride and Bacillus virens, Cohn's Micrococcus chlorinus, &c.) to the Alga'.

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 =1/.4) 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.

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 Begqiatoa roso-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 stai ding view that the various " form-species " and " form-genera " 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 Niigeli (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 1:nnwn of thr relntinne of iiIPSA organisms to fermentation 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 ificrococcus urea. 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 MVO in the special cases quoted. Nevertheless, instances were adduced where the most careful heating of yolk of egg, milk, hay-infusions, 4.4z,c. had failed, - the boiled infusions, &c., turn- ing 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. Bayer 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. Septicemia, tuberculosis, glanders, fowl-chole'ra, 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, 47.c., are now burning, - Motschnikoff'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, Niigeli, 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 modo of action of pathogenic Schizomycetes, as to the nature of immunity, and as to the limitation of "species" among such simple forms.1