3.

Take a drop of fluid from a turbid hay infusionand examine it, using the highest power you have; in it will be found multitudes of

Moving Bacteria.

4.

a.

b.

C.

d.

Note their

Form; elliptic or rodlike-sometimes forming short (2-8) jointed rows.

Size; breadth, very small but pretty constant; length, varying, but several times greater than their breadth: measure.

Structure; an outer more transparent layer enveloping less transparent matter: in the compound forms the envelope appears only where two joints come in contact, so that the rod looks as if made up of alternating transparent and more opaque substances.

Movements; some vital, and some purely physical (Brownian). The former various but progressive: the latter a rotatory movement round a stationary centre; study it in a drop of boiled infusion in which the Bacteria are all dead.

Treat with iodine-only the more opaque parts stain; probably then we have to do with protoplasm, enveloped in nonprotoplasmic matter.

5. Resting Bacteria. (Zooglea-stage.)

a.

Examine the scum from the surface of a hay infusion; it exhibits myriads of motionless Bacteria, embedded in gelatinous material.

b. Treat with iodine; the Bacteria stain as before: the gelatinous uniting material remains unstained.

Besides the form hitherto described, which would be known as Arthrobacterium, many other Schizomycetes will certainly be found, both in the pellicle and the fluid beneath. The following may be especially mentioned:

Micrococcus. Bodies much like Arthrobacterium but short and rounded, and occurring singly, or in bead-like rows. They may be found free or in a Zooglcea stage.

Bacillus. Threads composed of straight cylindrical joints much longer than those of Arthrobacterium, but of similar structure: they are always free-swimming.

Vibrio. Like Bacillus, but with bent joints.

Spirillum. Elongated unjointed threads rolled up into a more or less perfect spiral: frequently two spirals interwine. 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.

Examine various putrefying substances for different forms of Bacteria. Successful cultures may be made on hard-boiled white of egg, or slices of potato, kept moist under a bell-glass. Various brilliantly coloured micrococci including the bloodred "Micrococcus prodigiosus" will make their appearance on such cultures. An infusion of peaflour, filtered, is especially favourable for Spirochate.

8.

a.

b.

C.

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.

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.

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.

XIII.

MOULDS (Penicillium, Eurotium 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, except perhaps the Bacteria, are simpler in structure than Torula and Protococcus, and the only animals which are simpler than Amabæ, are essentially Amabæ 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 ; 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. We have already seen that the like is true of all the higher animals. They commence as nucleated cells, essentially similar to Amaba 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 contains 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 aërial 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 hypha 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