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other vegetable cells nuclei, or at least nuclear substance in the form of chromatic bodies, exist, though they are extremely minute; Fischer doubts their being true nuclei, but Migula accepts the view that they represent them in a rudimentary form. With regard to the essential details in the process of spore-formation and germination, resistance to physical reagents, &c., little advance in knowledge was Spores. macie iu decade 1890-1900. De Bary’s conception of arthrospores, as contrasted with endospores, has been found of little value; an arthrospore is merely an isolated bacterial cell in a resting state, and such occur in endosporous forms, e.[/., B. subtilis, as well as in species not known to form endospores. In a few cases, e.g., B. Zopfii, arthrospores appear to be well formed, but even in this typical case it is evident that we must regard these resting vegetative cells as in the same category as the oidia or gemmae of true fungi, and in no way morphologically specialized as spores. Similar objections have

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A- RM.

P.M. Pig. 3.—Stages in the development of spores of Bacillus ramosus (Praenkel), in the order and at the times given, in a hanging drop culture, under a very high power. The process begins with the formation of brilliant granules (A, B); these increase, and the brilliant substance gradually balls together (C) and forms the spores (D), one in each segment, which soon acquire a membrane and ripen (E). (Original.)

53 temperatures have also been used successfully. Most spores are ellipsoid, but those of B. tetani are spherical, and those of B. leptosporus elongated. The spore may be small and cause no deformation of the mother-cell, in which it lies at one end or in the middle; or it may during formation bring about a swelling of the cell or part of it, so that the mature sporogenous cell is drumstick-shaped (Plectridimn) as in B. tetani, or fusiform (Clostridiuui) as in C. butyricum, &e. Only in rare cases are two spores formed in the rod, e.g., B. injlatus. Much careful work has been done on the details of the formation, structure, and germination of spores, but little has been added to what has long been known, viz., that the spore results from aggregation of the protoplasmic contents of the rod (Fig. 3), that no definite nucleus can be made clear, that the spore-wall—probably of some altered protoplasmic product—is thin and extremely resistent when ripe, and that germination is preceded by the rupture or dissolution of the latter at the ends or equator, where it is apparently thinner, the contents growing out as the first rodlet (Fig. 2). Numerous attempts have been made to construct schemes of classification based on the power of growing colonies to liquefy gelatine, to secrete coloured pigments, to ferment certain media with evolution of carbondioxide or other gases, or to induce pathological conditions in animals. None of these systems, which are chiefly due to the medical bacteriologists, has maintained its position, owing to the difficulty of applying the characters and to the fact that such properties are physiological and liable to great fluctuations in culture, because a given organism may vary greatly in such respects according to its degree of vitality at the time, its age, the mode of nutrition, and the influence of external factors on its growth. Even when used in conjunction with purely morphological characters, these physiological properties are too variable to aid us in the discrimination of species and genera, and are apt to break down at critical periods. Among the more characteristic of these schemes adopted at various times may be mentioned those of Miquel (1891), Eisenberg (1891), and Lehmann and Neumann (1897). Although much progress was made during the decade 1890 to 1900 in determining the value and constancy of morphological characters, we are still in need of a sufficiently comprehensive and easily applied scheme of classification, partly owing to the existence in the literature of imperfectly described forms, the life-history of which is not yet known, or the microscopic characters of which have not been examined with sufficient accuracy and thoroughness. The principal attempts at morphological classifications recently brought forward are those of de Toni and Trevisan (1889), Fischer (1897), and Migula (1897). Of these systems, which alone are available in any practicable scheme of classification, the two most important and most modern are those of Fischer and Migula. The extended investigations of the former on the number and distribution of cilia (Fig. 1) led him to propose a scheme of classification based on these and other morphological characters, and differing essentially from any preceding one. This scheme may be tabulated as follows:—

been made to the use of the term arthrospore in the case of LeuconostoCy where true endospores appear to be formed. In short, the “ arthrospore ” is nothing more than any ordinary vegetative cell which separates and passes into a condition of rest—just as do the cells of yeast and the encysted cells of many micro-organisms—and according to the older definition every cell of a Micrococcus would be an “arthrospore.” The so-called arthrospores of Cladothrix and other chlamydo - bacteriaceae are ciliated motile cells, comparable with the zoospores of Algae, and germinating forthwith to new filaments. The conditions for spore-formation differ. Anaerobic species usually require little oxygen, but aerobic species a free supply. Each species has an optimum temperature, I. Order—Haplobacterinae. Vegetative body unicellular; spheroidal, cylindrical, or spirally twisted; and many are known to require very special food-media. isolated or connected in filamentous or other growth The systematic interference with these conditions has series. enabled bacteriologists to induce the development of so1. Family—Coccace.-e. Vegetative cells spheroidal. called asporogenous races, in which the formation of spores is indefinitely postponed, changes in vigour, viru(a) Sub-family—Allococcace^;. Division in all or any planes, colonies indefinite in shape and size, lence, and other properties being also involved, in some of cells in short chains, irregular clumps, pairs, cases at any rate. The addition of minute traces of acids, or isolated: — Micrococcus (Cohn), cells nonpoisons, &c., leads to this change in some forms; high motile ; Planococcus (Migula), cells motile.