cells of a tissue with the nutritive material which they absorb. In all the higher animals this nutritive material is the blood or lymph, but the products which are formed are not entirely the same for all cells, since they vary in some measure with the specific activity of the cell; thus the cells of the salivary glands yield the saliva, those of the mammary gland milk, and those of the liver form, besides other substances, glycogen. But all protoplasm, whatever may be its specific function, has this in common, viz., that it absorbs and combines with oxygen, and yields carbon dioxide and other products of oxidation, and as a result of these processes of oxidation heat and other kinds of energy are produced. These chemical changes are always more marked as the functional activity of the cell becomes increased, and accordingly any circumstances which tend to promote the activity of protoplasm, such as warmth, electrical or other stimulation, the action of certain drugs, tend proportionally to increase the activity of its chemical processes. One general chemical property of living protoplasm is that by virtue of which it is able to assimilate and eventually to convert into its own substance non-living proteid material. In this manner the protoplasm of a cell may increase in amount, or in other words the cell may grow; but if the amount of protoplasm does not permanently increase, this is due to the fact that just as much protoplasm is being broken down and removed from the cell as is added by the process of assimilation. Chemical processes which involve the building up of living material within a cell have received the general name of anabolic changes; those on the other hand which involve the breaking down of such material into other and simpler products are known as katabolic. By the metabolism of a cell is understood the sum of all the ana- and katabolic changes which are proceeding at any time within it.

Amoeboid movements.-The most obvious physical changes seen in living protoplasm are those which are designated "amoeboid." This term was derived from the freshwater amoeba, the protoplasm or sarcode of which has long been known to exhibit spontaneous changes of form, accompanied by a flowing or streaming of its soft semifluid substance. The phenomenon was in fact described by Rosenhof in 1755, but the similar movements of the cell-protoplasm of the higher animals was only recognised much later (in 1846 by Wharton Jones, who noticed the amoeboid movements of the white blood-corpuscles of fish). If the protoplasm of the cell is enclosed by a membrane its movements are necessarily confined within the limits of such cell-wall, and the actual changes which are in these cases observable consist in a streaming or flowing of the soft living substance, such flowing being rendered obvious by the carrying along by the stream of any minute particles which may be imbedded in the protoplasm. The term "rotation" has been given to a movement of this kind which is observed in many plant-cells, and is of a very regular character, and usually in a determinate direction; but in animal cells the intrinsic streaming movements are less regular and usually less obvious in character. It is, however, on the other hand in those animal cells which are unprovided with a cellwall (free or naked cells) that what may be termed the amoeboid movements proper present themselves, and in none more strikingly than in the pale blood- or lymph-corpuscles. If one of these be observed under a high power of the microscope it will be seen gradually to protrude a portion of its protoplasm at one part or another, and sometimes at several places simultaneously. These protrusions (pseudopodia) may be presently withdrawn again and others given out, or a pseudopodium which has been protruded at any one part of the corpuscle may extend itself further, and the main part or body of the corpuscle may pass gradually towards and into the extending pseudopodium. By a repetition of this process the cell may glide slowly, away from its original situation, and move bodily along the field of the microscope so that an actual locomotion thereby results. In this manner the white corpuscies may, even while the blood is circulating, pass through the walls of the capillaries and

minute veins and find their way into the surrounding connective tissue, where they may further continue to exhibit amoeboid movements. Corpuscles which have thus emigrated from the vessels are known as "wandering cells," and the process of emigration is known as "diapedesis." Although probably occurring to a certain extent normally, it is greatly increased in inflammatory conditions of the tissues.

Inception of foreign particles.-When any foreign particle comes in contact with a free cell, the particle adheres to it, becomes enwrapped by processes of the protoplasm, and is then drawn gradually into the interior, where it may remain for some time without change, being moved about by any currents which exist in the cell, and carried along by the changes of place which the cell undergoes (see fig. 204). Eventually such foreign particles may be extruded again. If, on the other hand, the particle is of considerable size as compared with the protoplasm with which it comes in contact, the latter extends around and over it so as to envelop it more or less completely. This phenomenon of inception seems thus to be dependent upon amoeboid movements of the protoplasm.

Conditions influencing the contractile manifestations of protoplasm.— All the several manifestations of contractility are influenced in the same manner by

Fig. 204. CHANGES OF FORM OF A WHITE CORPUSCLE OF NEWT'S BLOOD, SKETCHED AT INTERVALS OF A FEW MINUTES. THE FIGURES SHOW ALSO THE INTUSSUSCEPTION OF TWO SMALL GRANULES, AND THE CHANGES OF POSITION WHICH THESE UNDERWENT WITHIN THE CORPUSCLE. (E.A.S.)

similar external conditions. Thus it is found that variations of temperature have a marked effect upon all. In warm-blooded animals the phenomena cease altogether to be exhibited, if the protoplasm which is under observation is cooled to below a temperature of about 10° C., although they will be resumed on warming the preparation again, and this even if it has been cooled to 0° C., or a little lower. And when warmed gradually, it is found that the movements become more active as the temperature rises, attaining a maximum of activity a few degrees above the natural temperature of the body, although if maintained at an abnormally high temperature, they are not long continued. A temperature a little above this maximum, rapidly kills protoplasm, producing a stiffness or coagulation in it (heat-rigor) which is preceded by a general contraction. From the condition of rigor the protoplasm cannot be recovered.

The contractility of protoplasm is dependent upon supply of oxygen. If this be withheld, the movements will, it is true, proceed for a time as usual, but this is because protoplasm, like other forms of contractile substance, such as muscle, has the power of storing away and using oxygen in some form of combination. For it is found that the active manifestations will not proceed indefinitely in the absence of oxygen, but cease after a time, to be renewed only on the accession of fresh oxygen.

EFFECT OF ELECTRICAL AND OTHER STIMULI UPON PROTOPLASM. 177

Many reagents in solution influence the activity of protoplasm. Some of these act by adding to or subtracting from the water which it contains. As a general rule the imbibition of water up to a certain point, varying according to the source of the protoplasm which is under observation (Thoma), accelerates the activity of the protoplasm, but beyond that point addition of water produces a destructive effect. A comparatively slight amount of desiccation is, so far at least as regards the protoplasm of the higher animals, destructive of vitality, but this statement does not hold good for the protoplasm of many of the lower animal and plant organisms.

Amongst reagents acids, although very weak (even carbonic acid), stop the contractile manifestations; alkalies, on the other hand, if sufficiently dilute, increase at first their activity. They are stopped by chloroform and ether, but may be again resumed on the removal of those vapours. Some poisons (e.g., veratria) rapidly arrest the movements of cells.

Effect of electrical and other stimuli upon protoplasm.-The effect of electrical stimulation upon protoplasm which is exhibiting either amoeboid or streaming movement is, if sufficiently strong, to cause an immediate cessation of those movements, accompanied by a withdrawal into the main substance of any processes that may have been protruded. If the stimulation cease the movements will recommence, provided the shock has not been so severe as to injure the living substance.

Abrupt changes of temperature, and mechanical stimulation, such as is produced by sudden pressure or harsh contact, act in a similar manner.

Further considerations regarding the structure of protoplasm.-If the pseudopodia of a white blood-corpuscle are observed with a high power of the micro

Fig. 205.-AN AMEBOID PALE CORPUSCLE OF THE NEWT,

KILLED BY INSTANTANEOUS APPLICATION OF STEAM,
SHOWING THE STRUCTURELESS APPEARANCE OF THE

PSEUDOPODIA. (Drawn by D. Gunn.)

scope, they appear when first thrown out from the body of the corpuscle to be perfectly clear and homogeneous as if composed of hyaloplasm alone. Subsequently the reticular part of the protoplasm may flow into the pseudopodium. This structureless character of the pseudopodia can be well observed if a preparation of the

blood of the newt be made and set aside for half-an-hour or more until the white blood corpuscles have had time to throw out broad flattened pseudopodia which spread themselves over the under-surface of the cover-glass. If now a jet of steam be allowed to play for a moment upon the cover-glass, these corpuscles are instantly killed without having been able to withdraw the pseudopodia. They can then be hardened and stained, and observed with the highest powers of the microscope, when it is found that the thin extended portions of protoplasm show no trace of structure but appear completely homogeneous, in decided contrast with the main part of the cell, which exhibits distinctly the spongio-plasmic network.

Stricker has lately published a photograph of the white blood-corpuscle of a Proteus anguineus, which demonstrates the same fact. In this photograph, which was taken from the living amoeboid cell by an instantaneous method, the body of the cell, which is nearly spherical, is beautifully reticular, whilst a thin external layer and a pseudopodium, which is being protruded, appear completely homogeneous.

From this it is clear that the pseudopodial protoplasm, that namely to which the more obvious activity of the cell is immediately due, is structureless, and, probably, composed of hyaloplasm alone. The amoeboid movements are produced by a flowing of this hyaloplasm, which may extend, as in many Rhizopods, far beyond the limits of the spongioplasm. But if the corpuscle is subjected to artificial stimulation of any kind (electrical, mechanical, thermal) the pseudopodia are withdrawn into the body of the corpuscle, which then becomes spherical and appears wholly reticular. The spherical form of the corpuscle represents, therefore, the completely contracted condition; it is only in the absence of any obvious source of excitation that the cell throws out pseudopodia. Since the spherical condition is produced by mechanical stimuli, it is probable that the impacts which the white blood-corpuscles are constantly receiving in the circulating blood maintain them in the spherical form which they always exhibit within the vessels, and immediately the blood is drawn. It may also be that the inception of foreign particles by amoeboid cells is produced by the excitation which their contact produces upon the portion of hyaloplasm to which they adhere, causing its contraction and withdrawal into the spongioplasm.

To sum up :-The protoplasm of an amoeboid cell is composed of two substances -spongioplasm and hyaloplasm. The spongioplasm has a reticular appearance and sponge-like structure, an affinity for staining fluids, is firmer and more refractile than the hyaloplasm, and is probably highly extensile and elastic, but not actively contractile. The hyaloplasm, on the other hand, appears structureless, has little or no affinity for stains, and is highly labile and fluent. It is the active flowing movements of the hyaloplasm which produce the well-known phenomena of amoeboid activity. The spongioplasm forms a sort of framework which serves to support the hyaloplasm, and into which, under the influence of stimuli, the hyaloplasm may become entirely withdrawn. In non-amoeboid cells the hyaloplasm does not extend beyond the limits of the spongioplasm. The latter is the "coid," hyaloplasm the "zooid," to adopt terms which Bruecke has introduced into histology, although with a somewhat different signification (see p. 242).

The question has been frequently discussed whether we are to regard the spherical condition as that of rest, and the amoeboid condition as that of activity, or vice versa. Viewed by the light of the above statements it is clear that both are manifestations of activity, both being produced by flowing of hyaloplasm. In the one case this flows into the pores of the spongioplasm; this is the condition which corresponds to the contraction of muscle; in the other case the hyaloplasm flows out of the spongioplasm, producing a condition corresponding to the extension of muscle after contraction.2

Whether one or other of the two substances is ever wholly absent from the protoplasm of cells is a question which cannot at present be decided. There are cells and unicellular organisms, both animal and vegetable, in which no reticular structure can be made out, and these may be formed of hyaloplasm alone. In that case, this must be looked upon as the essential part of protoplasm. So far as amoeboid phenomena are concerned, it is certainly so; but whether the chemical changes which occur in many cells are effected by this or by spongioplasm is another matter.

The movements within plant cells must also be regarded as due to the flowing of hyaloplasm. It is, indeed, impossible to conceive that the contraction of a reticulum could produce the circulation of the protoplasm which is seen within a cell of Vallisneria. How the flowing is produced is an entirely different question, and one which must at present remain unanswered. Quincke has shown that if a drop of solution of albumin surrounded by an envelope of oil is placed in water a soapy film forms at the junction of the oil and water, and the drop

1 Carnoy and many other histologists have assumed that the spongioplasm or reticulum is the contractile part of protoplasm, and that the hyaloplasm is passive. If this were the case electrical excitation should cause the reticulum to shrink and to squeeze the hyaloplasm out of its meshes whereas the contrary is the case, the hyaloplasm passes into the meshes of the spongioplasm, which become thereby enlarged.

2 In dealing with the structure of muscle it will be shown that here also as in protoplasm there is a passage of a substance resembling hyaloplasm into and out of a porous spongioplasm. It will further be presently shown that the activity of cilia, can also be explained by assuming a flowing of the cell-hyaloplasm into and out of the cilia, so that all these forms of contractile phenomena can be brought into the same category.

exhibits changes of form due to alterations of surface tension, which are comparable to the amoeboid movements of living cells. Still more recently, Bütschli has found that if oil is rubbed up into a paste with certain alkaline salts in a moist condition, and some of the paste is examined in water, the latter diffuses into the paste and converts it into a froth, which, under the microscope, has an appearance not unlike the reticular part of protoplasm. In such a froth streaming movements may be seen, lasting for a considerable time, and changes of form may occur in the mass, due partly to continued diffusion through the soaplike envelopes of the froth-bubbles or vacuoles, and partly to the bursting of these bubbles when they become enlarged and approach the edge of the mass. Upon these observations Bütschli has based a theory that all protoplasm has such a vacuolated froth-like structure, that the reticulum is only apparent, being the optical expression of the material between the vacuoles, and that the movements of protoplasm are produced by physical and chemical processes analogous to those which cause the movements within the froth of oil and saltsolution which he has employed. Whilst admitting the interest of Bütschli's observations, it would, I think, be unwise to follow too far the deduction which he is inclined to draw from them. For amongst other important objections which might be urged, the absence of reticular appearance, and, therefore, of frothy structure from the active protoplasm of pseudopodia is in itself fatal to the theory. This difficulty Bütschli meets by assuming that such structure is really there although the thinning out of the pseudopodia has rendered it invisible. But the line of junction of the spongioplasm and hyaloplasm of an amoeboid cell is sharp (fig. 205), and shows no tendency to fade off gradually into the pseudopodia, as on Bütschli's assumption it should unquestionably do.

Nägeli, on theoretical grounds, has conceived that the essential living substance must in its ultimate (ultra-microscopical) structure consist of solid particles (or systems of such particles) surrounded by material of fluid consistence. To these hypothetical particles of living matter (or to the systems which they form) he has given the name micellæ (tagmata, Pfeiffer), and he supposes that they may, like the substances known as ferments, produce chemical changes in materials which are in contact with them without themselves undergoing any permanent or perceptible change (catalytic action).

THE NUCLEUS OF THE CELL.

The nucleus is a minute vesicular body, placed generally near the centre of the cell and embedded therefore in the protoplasm. In form it is round or oval in most cases, but it may be elongated and folded or irregular in shape. Its size relatively to that of the cell varies much in different instances, for sometimes there is so small an amount of protoplasm that the nucleus appears to occupy nearly the whole cell. This is the case with many of the cells which are met with in lymphatic glands, and with the small nerve-cells which are found in the cerebellum and elsewhere. On the other hand, the protoplasm of the cell, whether altered as in the superficial layers of some stratified epithelia, or unaltered as in many of the white corpuscles of the blood, may much exceed the nucleus in bulk. In absolute size, the nucleus does not exhibit so considerable a variation as do the cells of the same animal. There are, however, some notable exceptions; thus the nucleus is absolutely much larger in the ova and in many nerve-cells than it is in other cells of the body.

Structure of the nucleus.—In the typical "resting " condition the nucleus is always bounded by a well-defined wall, which encloses the nuclear contents. These are of two kinds, formed and amorphous. To the latter the term nuclear fluid is sometimes applied, and the former may be conveniently termed chromoplasm (nucleoplasm or karyoplasm of authors); this term is used to include also the substance which forms the wall of the nucleus. But it is by no means certain that the homogeneous amorphous substance which occupies the interstices of the chromoplasm is entirely fluid, so that it is better termed nuclear matrix. It is very possible that this homogeneous matrix of the nucleus may be of the same nature as the hyaloplasm of the cell-substance if indeed it is not actually continuous with it, but the chromoplasm is not the same as the spongioplasm of the cellsubstance.