Common Objects of the Microscope

A description of the fresh-water sponge already given may serve very well to indicate the general outlines of the organisation of the marine ones too. The spicules of the latter are, however, not always flinty; very often, as in the case of Grantia (Plate IX. Figs. 8 and 14), they are calcareous, a point which can be settled by the application of a little nitric acid and water. If lime be present there will be strong effervescence, and the separation of the spicules can only be effected by gently warming a portion of the sponge in caustic potash solution, pouring the resulting mass into water, and allowing the spicules to settle. The washing and settling must be repeated several times, and a portion of the deposit may then be taken up with a dipping-tube, spread upon a slide and dried, and then covered in balsam solution. The forms are endless, and the same sponge will often supply three or four, or even more. Among them may be seen accurate likenesses of pins, needles, marlin-spikes, cucumbers, grappling-hooks, fish-hooks, porters’-hooks, calthrops, knife-rests, fish-spears, barbed arrows, spiked globes, war-clubs, boomerangs, life-preservers, and many other indescribable forms. The flinty forms must be prepared by boiling, as described in speaking of the mounting of diatoms in Chapter XI., except that, of course, only one settlement is required after thorough washing.

Every one who has been by or on the sea on a fine summer night must have noticed the bright flashes of light that appear whenever its surface is disturbed; the wake of a boat, for example, leaving a luminous track as far as the eye can reach. This phosphorescence is caused by many animals resident in the sea, but chiefly by the little creature represented at Fig. 9, the Noctilúca, myriads of which may be found in a pail of water dipped at random from the glowing waves. A tooth of this creature more magnified is shown immediately above.

A large group of microscopic organisms is known to zoologists under the name of Foraminifera, on account of the numerous holes in their beautiful shells, most of which are composed of carbonate of lime, though some are horny and others are composed of aggregations of minute grains of sand, the forms in one class often closely imitating those in another. It is of the shells of these minute animals that the “white cliffs of old England” are very largely composed, and those who desire to understand the part which these tiny creatures have played, and are playing, in geology, will do well to study Huxley’s fascinating essay on “A Piece of Chalk.”

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The inhabitants of these shells are Amœbæ, mere masses of jelly, and some forms may be found sliding over the weeds in almost every rock-pool. The anchor-mud, already spoken of, always contains some, and many forms may be found in the sand from sponges, which should be passed through a series of wire sieves, of increasing fineness, and the residuum in each case be examined dry under a one-inch power. The shells may be picked up with a needle which has been slightly greased by being passed over the hair, and they may be mounted by sticking them to the slide with thin starch paste, putting on a cover-glass properly supported, and then running turpentine under the cover-glass, heating to expel the air, and finally filling up with balsam. Or, as opaque objects, they may be mounted in a cell dry. The forms are endless, but all are beautiful, and a few examples are given in Plate IX. Fig. 4 (Miliolína), and Plate XII. Fig. 7, which is a portion of the shell to show the holes, Fig. 13 (Polystomella), Fig. 14 (Truncatulína), Fig. 15 (Polymorphína), Fig. 16 (Miliolína, partly fossilised), Fig. 18 (Lagéna). and Fig. 20 (Biloculína).

Allied to these are the lovely Radiolaria, whose shells, constructed on a similar plan, are composed of flint. They are found in remarkable profusion in the deposit from Cambridge, Barbados, but also in a living state at even enormous depths in the ocean. The present writer has obtained many forms from Challenger soundings, and the great authority on this subject is Haeckel’s report in the official accounts of the expedition of the above-named vessel.

The Hydroid Zoophytes are represented by several examples. These creatures are soft and almost gelatinous, and are furnished with tentacles or lobes by which they can catch and retain their prey. In order to support their tender structure they are endowed with a horny skeleton, sometimes outside and sometimes inside them, which is called the polypidom. They are very common on our coasts, where they may be found thrown on the shore, or may be dredged up from the deeper portions of the sea.

Fig. 13 is a portion of one of the commonest genera, Sertularia, showing one of the inhabitants projecting its tentacles from its domicile. Fig. 15 is the same species, given to show the egg-cells. This, as well as other zoophytes, is generally classed among the sea-weeds in the shops that throng all watering-places.

The form just referred to is a near relative of the Hydra, already described, and belongs to the same great family as the sea-anemones. One form, shown in Fig. 26, is the Hydra Tuba, long thought to be a distinct animal, but now known to be the young form of a jelly-fish, or Medusa. The Hydra Tuba throws off joints at intervals, each of which becomes a perfect jelly-fish. One of them is shown in Fig. 27. Fig. 28 represents a very small and pretty Medusa, the Thaumantias. When this animal is touched or startled, each of the purple globules round the edge flashes into light, producing a most beautiful and singular appearance. Fig. 29 exhibits the so-called compound eye of another species of Medusa, though it would appear that these are really connected with the nervous system of the animal, and have to do with the pulsating contractions of the bell by which it is propelled through the water.

In my Common Objects of the Sea-Shore the Actíniæ, or Sea-Anemones, are treated of at some length. At Fig. 16 is shown part of a tentacle flinging out the poison-darts by which it secures its prey; and Fig. 17 is a more magnified view of one of these darts and its case.

Much more might be said under this head, but we must pass on to another group, which, whilst possessing a certain general resemblance to the hydroid zoophytes, differs utterly from them in internal organisation. We have already referred to the fresh-water polyzoa. The marine forms are vastly more numerous, and more easily found, since not only pieces of weed upon which they grow are to be found upon every beach, but whole masses of leaf-like colonies, forming what is known as horn-wrack, may be plentifully found. Instead of the tentacles armed with sting-cells, like the anemone’s, possessed by the Hydrozoa, the Polyzoa have arms clothed with active cilia, by which the food is swept into the mouth, passing on into the stomach, and then through the intestine to another opening.

Fig. 19 is a very curious zoophyte called Anguinaria, or snake-head, on account of its shape, the end of the polypidom resembling the head of the snake, and the tentacles looking like its tongue as they are thrust forward and rapidly withdrawn. Fig. 21 is the same creature on an enlarged scale, and just below is one of its tentacles still more magnified. Fig. 23 is the ladies’-slipper zoophyte (Eretea); and Fig. 24 is called the tobacco-pipe or shepherd’s-purse zoophyte (Notamia).

Fig. 22 is a portion of the Bugula, with one of the curious “birds’-head” processes. These appendages have the most absurd likeness to a bird’s head, the beak opening and shutting with a smart snap (so smart, indeed, that the ear instinctively tries to catch the sound), and the head nodding backward and forward just as if the bird were pecking up its food. On Plate XII. Fig. 2, is a pretty zoophyte called Gemellaria, on account of the double or twin-like form of the cells; and Fig. 5 represents the Antennularia, so called on account of its resemblance to the antennæ of an insect. Fig. 22 is an example of a pretty zoophyte found parasitic on many sea-weeds, and known by the name of Membranipora. Two more specimens of zoophytes may be seen on Plate XI. as they appear under polarised light. Fig. 17 is the Cellularia reptans; and Fig. 20 is the Bowerbankia, one form of which occurs in fresh water.

Among the worms we may refer to the beautiful little Spirorbis, whose tiny coiled spiral tube may be found attached to almost every sea-weed, and which, when placed in a trough of sea-water, protrudes its beautiful crown of plumes. In chalk or other soft rocks, again, the tubes of Spio, with its two long waving tentacles, may be found by hundreds. Then there are the centipede-like worms, which may be found under nearly every stone, and which belong to the great family of Nereids, provided with formidable jaws and stiff bristles of various forms. The Serpulæ are allied to the Spirorbis already mentioned. Parts of the so-called feet of one of these worms are shown in Fig. 36, where the spears or “pushing-poles” are seen gathered into bundles, as during life. One of them, on a larger scale, is shown in Fig. 32. The gorgeous hairs of Aphrodite have already been alluded to.

In the sea the few species of Crustacea which fresh water offers to the observer in the shape of Cyclops and its allies become thousands, and their changes during development are numerous and puzzling. Who, for example, would suppose that the young stage of the Cyclops was indistinguishable in habits, and almost in form, from that of the barnacle which adheres to the rocks? Yet such is the case, and there are other metamorphoses even more startling. Fig. 25 is the larva of the common crab, once thought to be a separate species, and described as such under the name of Zoæa.

The Mollusca proper will not afford us many objects, except in the form of their lingual ribbon, which may be extracted from the mouth, gently heated in liquor potassæ, and mounted in balsam after well washing, when the rows of teeth form splendid objects by polarised light. The palate of a whelk is shown in Plate XI. Fig. 19.

Again, the gills of the mussel will afford a beautiful illustration of ciliary action. If a portion of the thin plates which lie along the edge of the shell be examined in a little of the liquor, the action may be splendidly seen, and watched for a long time (Fig. 39).

The structure of shell, e.g. oyster-shell, is well shown in three examples: Fig. 34 is a group of artificial crystals of carbonate of lime; and on Figs. 38 and 39 may be seen part of an oyster-shell, showing how it is composed of similar crystals aggregated together. Their appearance under polarised light may be seen on Plate XI. Figs. 1 and 6.

We now pass on to the Echinoderms, including the star-fishes and sea-urchins.

The old story of the goose-bearing tree is an example of how truth may be stranger than fiction. For if the fable had said that the mother goose laid eggs which grew into trees, budded and flowered, and then produced new geese, it would not have been one whit a stranger tale than the truth. Plate IX. Fig. 33, shows the young state of one of the common star-fishes (Comátula), which in its early days is like a plant with a stalk, but afterwards breaks loose and becomes the wandering sea-star which we all know so well. In this process there is just the reverse of that which characterises the barnacles and sponges, where the young are at first free and then become fixed for the remainder of their lives. Fig. 30 is the young of another kind of star-fish, the long-armed Ophiúris, or snake-star.

Fig. 37 is a portion of the skin of the common sun-star (Solaster), showing the single large spine surrounded by a circle of smaller spines, supposed to be organs of touch, together with two or three of the curious appendages called pedicellariæ. These are found on star-fishes and Echini, and bear a close resemblance in many respects to the bird-head appendages of the zoophytes. They are fixed on foot-stalks, some very long and others very short, and have jaws which open and shut regularly. Their use is doubtful, unless it be to act as police, and by their continual movements to prevent the spores of algæ, or the young of various marine animals, from effecting a lodgment on the skin. A group, of pedicellariæ from a star-fish is shown on a large scale on Plate XII. Fig. 6, and Fig. 9 of the same Plate shows the pedicellariæ of the Echinus.

Upon the exterior of the Echini, or sea-urchins, are a vast number of spines having a very beautiful structure, as may be seen by Fig. 35, Plate IX., which is part of a transverse section of one of these spines. An entire spine is shown on Plate XII. Fig. 12, and shows the ball-and-socket joint on which it moves, and the membranous muscle that moves it. Fig. 8 is the disc of the snake-star as seen from below. Fig. 1 is a portion of skin of the sun-star, to show one of the curious madrepore-like tubercles which are found upon this common star-fish. Fig. 3 is a portion of cuttle “bone,” very slightly magnified, in order to show the beautiful pillar-like form of its structure; and Fig. 4 is the same object seen from above. When ground very thin this is a magnificent object for the polariscope.

One or two miscellaneous objects now come before our notice. Fig. 11 is one of those curious marine plants, the Corallines, which are remarkable for depositing a large amount of chalky matter among their tissues, so as to leave a complete cast in white chalk when the coloured living portion of the plant dies. The species of this example is Jania rubens.

Fig. 19 is part of the pouch-like inflation of the skin, and the hairs found upon the rat’s tail, which is a curious object as bearing so close a similitude to Fig. 22, the sea-mat zoophyte. Fig. 23 is a portion of the skin taken from the finger, which has been injected with a coloured preparation in order to show the manner in which the minute blood-vessels or “capillaries” are distributed; and Fig. 26 is a portion of a frog’s lung, also injected.

The process of injection is a rather difficult one, and requires considerable anatomical knowledge. The principle is simple enough, being merely to fill the blood-vessels with a coloured substance, so as to exhibit their form as they appear while distended with blood during the life of the animal. It sometimes happens that when an animal is killed suddenly without effusion of blood, as is often seen in the case of a mouse caught in a spring trap, the minute vessels of the lungs and other organs become so filled with coagulated blood as to form what is called a natural injection, ready for the microscope.

Before leaving the subject I must ask the reader to refer again for a moment to the frog’s foot on Plate X., and to notice the arrangement of the dark pigment spots. It is well known that when frogs live in a clear sandy pond, well exposed to the rays of the sun, their skins are bright yellow, and that when their residence is in a shady locality, especially if sheltered by heavy overhanging banks, they are of a deep blackish-brown colour. Moreover, under the influence of fear they will often change colour instantaneously. The cause of this curious fact is explained by the microscope.

Under the effects of sunlight the pigment granules are gathered together into small rounded spots, as seen on the left hand of the figure, leaving the skin of its own bright yellow hue. In the shade the pigment granules spread themselves so as to cover almost the entire skin and to produce the dark brown colour. In the intermediate state they assume the bold stellate form in which they are shown on the right hand of the round spots. Very remarkable forms of these cells may be found in the skin of the cuttle-fish.

Figs. 24 and 25 are two examples of coal, the former being a longitudinal and the latter a transverse section, given in order to show its woody character. Fig. 17 is a specimen of gold-dust intermixed with crystals of quartz sand, brought from Australia; and Fig. 21 is a small piece of copper-ore.

Every possessor of a microscope should, as soon as he can afford it, add to his instrument the beautiful apparatus for polarising light. The optical explanation of this phenomenon is far too abstruse for these pages, but the practical application of the apparatus is very simple. It consists of two prisms, one of which, called the polariser, is fastened by a catch just below the stage; and the other, called an analyser, is placed above the eye-piece. In order to aid those bodies whose polarising powers are but weak, a thin plate of selenite is generally placed on the stage immediately below the object. The colours exhibited by this instrument are gorgeous in the extreme, as may be seen by Plate XI., which affords a most feeble representation of the glowing tints exhibited by the objects there depicted. The value of the polariser is very great, as it often enables observers to distinguish, by means of their different polarising properties, one class of objects from another.

If the expense of a polarising apparatus be too great for the means of the microscopist, he may manufacture a substitute for it by taking several thin plates of glass, arranging them in a paper tube so that the light may meet the surface of the lowest one at an angle of about 52°, and placing the bundle above the eye-piece to act as an analyser; whilst, by using a plate of glass, and so arranging the lamp that the light falls upon it at the above angle, and is reflected up the tube of the microscope, he will find on rotating the extemporised analyser that the phenomena of polarisation are to a great extent reproduced; whilst by splitting an extremely thin film from the surface of a sheet of mica, such as is employed for making smoke-screens above glass globes, he will have a substitute for the selenite by means of which alone can the gorgeous effects be produced. The extemporised apparatus will not, of course, give such perfect effects, but this is sometimes an advantage, and the present writer has used the same means with considerable success in photographing starch-granules.

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CHAPTER XI

Hints on the Preparation of Objects—Preservative Fluids—Mounting Media—Treatment of Special Objects.

The microscopist who relies altogether on the dealer for his permanent preparations may expend a good deal of money, but the satisfaction which he derives from his hobby will be very inferior to that experienced by the worker who endeavours to secure, for exhibition or for reference, specimens of the objects which he finds most interesting and instructive to himself.

It will be our endeavour in the following pages to give a summary of the elementary principles upon which reliance is to be placed, though it must be clearly understood that the technique of the subject, already occupying a vast amount of literature, is extending day by day, so that it is impossible to deal exhaustively even with one single section of it. Reference must be made, for further information, to such publications as the Journal of the Royal Microscopical Society, or that of the Quekett Club, or to the monographs on the various departments. Davies’ work on the general subject will also be found useful by the beginner.

Taking first the question of reagents, we may mention five which leave the cells of a tissue as nearly as possible in the natural condition, but fit for permanent preservation. The first of these, in order of importance and of general applicability, is alcohol, represented for most purposes by methylated spirit, which contains about 84 per cent. of absolute alcohol, though, unfortunately for our purpose, there is a certain quantity of mineral naphtha in it in addition. This last has the effect of making it go milky upon dilution with water, which is a considerable disadvantage, though the milkiness disappears to some extent on standing, and it is rarely worth the while of the ordinary microscopist to go through the formalities necessary to obtain permission to purchase unmineralised spirit, which cannot be had in quantities of less than five gallons (as it is only to be had from the distillers under an Excise permit), and distillers may not supply less.

Four parts of methylated spirit with one of water forms the classical “70 per cent.” alcohol, the most generally useful of all fluids for hardening and preserving purposes. A considerable quantity of this fluid should always be available.

Whatever other fluid may be used to begin with, spirit must almost always be used to finish the process, and fit the tissue for section-cutting and staining.

Of purely preservative, or fixative, fluids, we may mention “formalin,” a 40 per cent. solution of formic aldehyde, which is rapidly coming to the front, as indeed it deserves to do. It is but slightly poisonous, if at all, and leaves in the tissue nothing which requires subsequent removal before proceeding to harden for section-work, whilst it is an admirable preservative of cell-form.

Another admirable but highly poisonous reagent is corrosive sublimate, in saturated solution, with 2 per cent. of acetic acid.

A fourth is osmic acid, used in 1 per cent. solution. This is a highly valuable reagent, but extremely expensive, very poisonous, and giving off fumes which are most irritating to the eyes.

The fifth, a very gentle, but in many respects very satisfactory one, is picric acid in saturated solution. Tissues preserved in this medium must not be washed out with water, as it enters into very feeble combination with protoplasm, and the cells swell and disintegrate as the reagent is dissolved out.

Of mounting media we may mention glycerine, glycerine jelly (made by dissolving starch in glycerine with the aid of heat), and Canada balsam, dissolved in xylol or benzole. The Canada balsam must be dried hard by evaporation over a water-bath, and dissolved as wanted. Under no circumstances should raw balsam be used, as it takes years to set hard, and turns of a deep yellow colour in the process.

Chloroform is frequently used as a solvent, but it has the disadvantage of attacking and extracting a large number of the aniline dyes used for staining structures, an objection from which the mineral solvents are free.

We will now proceed to go through the objects already referred to, and indicate the method of preservation.

For the study of the cell-structures of plants the portion to be examined is to be placed in spirit of about 30 per cent. strength, which is changed after twenty-four hours for 40 per cent., after a further twenty-four hours for 55 per cent., and finally, as regards our present purpose, in 70 per cent. spirit, in which it may remain until required for section-cutting. The effect of this treatment is to extract the bulk of the water from the tissue, with the minimum of shrinkage of the cells, the latter being preserved in their natural relations to surrounding parts.

In some cases, however, it is desirable to examine and preserve delicate structures, or parts, or dissections, in a medium which allows of the retention of the greater part of the natural moisture, and in such a case the tissue is immersed in glycerine diluted very much in the same way as the alcohol in the last process, but with very much longer intervals between the alterations of strength, until it reaches pure glycerine, in which it remains for a considerable time, as the exchange between the tissue and the dense fluid surrounding it goes on very slowly.

A combination of the two methods is also possible, the spirit-hardening being carried out for a portion of the time, and the tissue being thereafter transferred to glycerine, diluted or pure.

The object of using glycerine at all is merely that it has a much lower refractive index than balsam, so that delicate structures may sometimes be better seen in the former medium, but balsam is to be preferred wherever it is possible to use it, i.e. almost always. The writer has not mounted a preparation in glycerine or a medium containing it for many years, nor, with proper staining, does he think it can ever be necessary to do so, except in the case of dissections in which the glycerine can be slowly run in without disturbing the arrangement, as spirit would be pretty sure to do. The harder portions of plants, woody stems, shells of fruit, or the like, require different treatment, and must, as a rule, be allowed to dry thoroughly before being cut.