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d.

e.

but of a similar structure: they are always freeswimming.

Vibrio. Like Bacillus, but with bent joints. Spirillum. Elongated unjointed threads rolled up into a more or less perfect spiral: frequently two spirals intertwine. In some of the largest forms a vibratile cilium can be made out on each end of the thread.

Spirochete. Much like spirillum, but longer and with a much more closely rolled spiral. A very actively motile but not common form.

7. Examine various putrefying fluids for Bacteria and related organisms.

8. Place some fresh-made hay infusion in three flasks; boil two of them for three or four minutes, and while one is boiling briskly stop its neck with a plug of cotton-wool and continue to boil for a minute or two: leave the necks of the other two flasks unclosed, and put all three away in a warm place.

a.

b.

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In a day or two abundant Bacteria will be found in the unboiled flask.

In the boiled but unclosed flask Bacteria will also appear, but perhaps not quite so soon as in.a.

C. In the flask which has been boiled and kept closed Bacteria will not appear, if the experiment has been properly performed, even if it be kept for many months.

V.

MOULDS (Penicillium and Mucor).

Torula, Protococcus and Amaba are extremely simple conditions of the two great kinds of living matter which are known as Plants and Animals. No plants are simpler in structure than Torula and Protococcus, and the only animals which are simpler than Amabæ, are essentially Amæbæ devoid of a nucleus and contractile vesicle. Moreover, however complicated in structure one of the higher plants may be in its adult state, when it commences its existence it is as simple as Torula or Protococcus, or at most as Torula or Protococcus would be if it possessed a distinct nucleus; and the whole plant is built up by the fissive multiplication of the simple cell in which it takes its origin, and by the subsequent growth and metamorphosis of the cells thus produced. The like is true of all the higher animals. They commence as nucleated cells, essentially similar to Amœbæ and colourless blood-corpuscles, and their bodies are constructed by aggregations of metamorphosed cells, produced by division. from the primary cell. It has been seen that Torula and Protococcus, similar as they are in structure, are distinguished by certain important physiological peculiarities; and the more complicated plants are divisible into two series, one produced by the growth and modification of cells which have the physiological peculiarities of Torula and contain no chlorophyll, while the other, and far larger, series

presents chlorophyll, and has the physiological peculiarities of Protococcus. The former series comprises the Fungi, the latter all other plants; only a few parasitic forms among these being devoid of chlorophyll.

The Fungi take their origin in spores, a kind of cells, which, however much they may vary in the details of their structure, are essentially similar to Torula. Indirectly or directly, the spore gives rise to a long tubular filament, which is termed a hypha, and out of these hyphæ the Fungus is built up.

One of the commonest Moulds, the Penicillium glaucum, which is familiar to every one from its forming sage-green crusts upon bread, jam, old boots, &c. affords an excellent and easily studied example of a Fungus. When examined with a magnifying glass, the green appearance is seen to be due, in great measure, to a very fine powder which is detached from the surface of the mould by the slightest touch. Beneath this lies a felt-work of delicate tubular filaments, the hyphæ, forming a crust like so much blotting-paper, which is the mycelium. From the free surface of the crust innumerable hyphæ project into the air and bear the green powder. These are the aerial hypha. On the other hand, the attached surface gives rise to a like multitude of longer branched hyphæ, which project into the fluid in which the crust is growing, like so many roots, and may be called the submerged hypha. If the patch of Penicillium has but a small extent relatively to the surface on which it lies, multitudes of silvery hyphæ will be seen radiating from its periphery and giving off many submerged, but few or no vertical, or subaërial, branches. Submitted to microscopic examination, a hypha is seen to be composed of a transparent wall (which has the same characters as the cell-wall of Torula) and protoplasmic contents, which fill the tube

formed by the wall, and present large central clear spaces, or vacuoles. At intervals, transverse partitions, continuous with the walls of the tube, divide it into elongated cells, each of which contains a correspondingly elongated protoplasmic sac, or primordial utricle. The hyphæ frequently branch dichotomously; and, in the crust, they are inextricably entangled with one another; but every hypha, with its branches, is quite distinct from every other. Those aerial hypha which are nearest the periphery of the crust end in simple rounded extremities; but the others terminate in brushes of short branches, and each of these branches, as it grows and elongates, becomes divided by transverse constrictions into a series of rounded spores arranged like a row of beads. The spores formed in this manner are termed conidia. At the free end of each filament of the brush the conidia become very loosely adherent, and constitute the green powdery matter to which reference has been made. Examined separately, a conidium is seen to be a spherical body, composed of a transparent sac, enclosing a minute mass of protoplasm, in all essential respects similar to a Torula. If sown in an appropriate medium, as for example Pasteur's solution, with or without sugar, the conidium germinates. Upon from one to four points of its surface an elevation or bulging of the cell-wall and of its contained protoplasm appears. This rapidly increases in length, and, continually growing at its free end, gives rise to a hypha, so that the young Penicillium assumes the form of a star, each ray being a hypha. The hyphae elongate, while side branches are developed from them by outgrowths of their walls; and this process is repeated by the branches, until the hyphæ proceeding from a single conidium may cover a wide circular area, as a patch of mycelium. When, as is usually the case, many conidia germinate close together,

their hyphæ cross one another, interlace, and give rise to a papyraceous crust. After the hyphæ have attained a certain length, the protoplasm divides at intervals, and transverse septa are formed between the masses thus divided off from one another. But neither in this, nor in any other Fungus, are septa formed in the direction of the length of the hypha.

Very early in the course of the development of the mycelium, branches of the. hyphæ extend downwards into the medium on which the mycelium grows; while, as soon as the patch has attained a certain size, the hyphæ in its centre give off vertical aerial branches, and the development of these goes on, extending from the centre to the periphery. The outgrowth of pencil-like bunches of branches at the end of these takes place in the same order; and these branches, becoming transversely constricted as fast as they are formed, break up into conidia, which are ready to go through the same course of development.

The conidia may be kept for a very long time in the dry state, without their readiness to germinate being in any way impaired, and their extreme minuteness and levity enable them to be dispersed and carried about by the slightest currents of air. The persistence of their vitality is subject to nearly the same conditions of temperature as that of yeast. Not unfrequently Torule make their appearance, in abundance, among the hyphæ and conidia of Penicillium, and appear to be derived from them; but it is still a disputed point, whether they are so or not.

If some fresh horse-dung be placed in a jar and kept moderately warm, its surface will, in two or three days, be covered with white cottony filaments, many of which rise vertically into the air, and end in rounded heads, so that

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