Page images

On every hand do we find objects of entrancing interest, new and never-ending wonders. The mind has hardly digested, if we may use the expression, one amazement, ere another, and perchance a still greater, breaks in upon it.

There are other sciences, indeed, which are not without much to attract and fascinate, but to our apprehension they pale like the ineffectual lights that man's necessities have compelled him to seek out, before the returning splendors of the god of day, in the comparison with this study of physiology.

There is geology, a science not lightly to be spoken against, but whose interest for the most part consists in the revelations of the life that has been, rather than in showing to us the crumbling fragments and broken remains, that preach to us of this last generation, our own oncoming decay.

There is chemistry, a science that scintillates along its pathway with a brilliancy that seduces while it dazzles the spectator, but whose not least useful service to the cause of mankind is, that it has been and now is the handmaid of physiology, in its endeavors to solve the problem of life. But we will not go on to summon all other instances; suffice it to say, that some sciences may tell us of the operation of unknown forces of unestimated power, upon the world of unanimated maiter; how through unknown ages these forces have been racking this old earth too and fro like a very toy-here lifting the mountain, and there sinking the valley-yonder bidding the sullen ocean retreat, and there thrusting it on the dry land : other sciences may tell us what is the composition of this unseen fluid that we breathe, on whose purity our lives depend, whose gentle cooling motion now sends a thrill of pleasure through the wearied and heated body, and whose power anon shall dash the mighty forest to earth, and make havoc with the labors of multitudes, and the trophies of centuries; they may analyze all minerals and earths; they may burn the diamond, and convert the elements of water into fire; other sciences still,

may bring out on the clear forehead of the night a thousand stars where was but one, may tell their distance and proclaim their number; but however interesting or attractive they may be, they deal with matter, unimpowered with life.

Physiology does indeed deal with matter, but only with matter as it is organized-vitalized-to which the power of triumphing over other matter, by bringing it to its uses and neces. sities

, has been delegated; which in the exercise of the prerogative of life resists the universal tendency of matter to decay and decomposition. It is the exercise of this prerogative that invests the humblest organism with that profound interest with


[ocr errors]

which the philosophic physiologist is accustomed to regard it. He sees in it the exercise of the functions of organized beings in their lowest and simplest forms. It furnishes him a starting point, from which his inquiries may rise by nicely graduated steps from one position to another, until he has traversed the whole scale of life-development; holding on, through all his researches, to the knowledge it has given him, as a key to unlock many an otherwise inscrutable mystery.

And it is this fact, we repeat, which gives to this science its enchantment: it has to do with life. We do not propose in the present article, to enter upon a review of the whole subject, as it is laid before us in the portly volume which heads this paper ; that evidently would be proper only in a scientific journal. What

What we do propose, however, is simply to present an intelligible view of one branch, viz: a brief history of the cell-development; a subject which when stripped of its technicalities as far as possible, will not fail, we think, to interest the minds of all intelligent readers, whose avocations may not have lead them along this line of scientific knowledge, and who would gladly be informed of the results of the researches of labors in other portions of the wide field of human knowledge. With this view we shall detail as succinctly as possible, the present state of our knowledge concerning the nature and functions of the cell. We ought, however, to premise, that such are the intrinsic difficulties attending the investigation, and so recently has the true method been adopted, that some points concerning the origin and development of cells have not been fully decided : in such instances we follow the views that are supported by the greatest force of authority. In this compilation we have used very freely the admirable work of Dr. Carpenter, and all other authorities open to us.

We make this general acknowledgment, to avoid breaking up the continuity of the article by repeated references.

We commence with what appears to be the true point of departure, viz: the simplest vegetable or plant, which is a cell, unconnected with any other, maintaining an independent life and propagating its kind. Every organized being has its origin from a cell, and hence the interest of the cell to the physiologist. Although it might seem a very insignificant matter, this microscopic cell, yet it is in reality a plant, and fulfills, in itself, the functions and powers of independent existence, and presents an epitome of the powers and functions of all other physical existences.

A cell, in the language of physiology, is a closed vesicle or minute bag, formed by a membrane in which no very definite struc


ture can be discerned, and having a cavity that may contain matter of variable consistence. Every such cell, as we have said, constitutes an entire organism in such simple plants as that known under the name of red snow, (Protococcus nivalis ;) and although this kind of vegetation occurs in patches containing vast numbers of such cells, yet they have no dependence on each other, and the actions of one are the repetition of all

[ocr errors]

the rest.

The cell generally takes its origin from a germ, which may be a molecule, so very minute as only to be visible under a microscope of high power. This germ, in its earliest condition, seems to be a simple homogeneous particle, and is spherical in form; as it gradually increases in size we are able to distinguish between the transparent exterior and the colored interior, which gives the first indication of the cell-wall and cavity. As the cell enlarges, the distinction becomes more obvious; the cell-wall is seen to be of extreme tenuity, perfectly transparent, and homogeneous in its texture ; while the contents of the cavity are distinguished by their color, red or green, according to the species. At first they too seem to be homogeneous, but a finely granular appearance is then perceptible, and a change gradually takes place, which seems to consist in the aggregations of the minute granules, into molecules of greater size. These molecules are the germs of new cells, and seem at first attached to the wall of the parent-cell ; afterward they separate from it, and move about in its cavity; and still later the parent-cell bursts and sets them free ; and thus the life of the parent-cell is terminated; but only as it fulfills the law of its being, and originates the commencement of new life, since every one of these germs will in its turn develop into a cell, and perpetuate its kind by the same process.

Such, however, is not the invariable method of propagation. A cell once originated in whatever manner, has the power of propagating itself by division into parts, each of which forms a new cell. Of this process there are two modifications. In the first mode, the cell is propagated by division into two, four, and sometimes a greater number of new cells. The mucilaginous lining of the cell-wall, (called the primordial utricle) becomes constricted or enfolded about the middle, and the fold extends inward until it is divided, with the whole contents into two parts; the two new cells thus produced may at once divide again, in the same way giving rise to four cells in a parent-cell ; or the division may be again and again repeated. In the second, the cell is multiplied by the formation of a partition, which divides its cavity into two, the original cell-wall remaining. In this way, a single cell gives rise to a row of connected cells, when it takes place in one direction only, or a plane or a solid mass of such cells when it takes place in two or more directions.

Let us now for a moment dwell on some of the endowments which the cell possesses, and see whether we were too bold in affirming that the simple cell presents us with a type of organized structure. · These endowments are extremely various, and constitute the differences between the properties of the several tissues. Thus we find certain cells whose agency is exerted in developing a preliminary organization or vitalization in the liquid pabulum, which when prepared is to be yielded up for the appropriation of other cells in the process of development. This process is designated by the term, Assimilation. There are other cells, which are distinguished by their power of reproduction ; their functions as individuals being apparently confined to the preparation of the germs of new generations. Others, again, are characterized by the charge of form which they undergo, after having attained their development as cells; this change of form, rendering them subservient to some new purposes in the economy, originates entirely in their

own vital activity, and indicates a power of Morphological Transformation. In other instances, the endowments of the cells are manifested in their power of effecting Chemical Transformation. And in both animals and plants we find a number of cells endowed with the power of giving rise to mechanical motion. Others, still, in the animal body generate that peculiar power known as nervous agency. Such are some of the widely varied powers or endowments of the cell.

But, we must pass on to a more particular description of the vegetable and animal cells; and also to point out in what essential points they differ. We have spoken of the cell-wall

, as a simple membrane ; but it is now generally known to be made up, in most instances at least, of two layers of very different composition and properties ; the inner of these layers, as we have said, is called the “primordial utricle,” and appears to be the first formed, and most essential to the existence of the cell. Its extreme thinness and delicacy will cause it to escape observation, so long as it remains in contact with the external layer ; but by proper manipulation it can be separated from that ; it is supposed to be an azotized compound, of an albuminous nature, and to take an active part in the vital operations of the cell. The external layer on the other hand, though commonly regarded

[ocr errors]

as the proper "cell-wall,” seems to be generated on the external

" surface of the primordial utricle, after the cavity is completely enclosed by the latter. It does not appear to take any active share in the vital operations of the cell; its principal office being to deposit, protect, and isolate the matter it contains. It is readily permeable to fluid, though no pores have been detected in it, under the highest magnifying power.

The granular matter in the interior of the cell is usually colored, and has received the name of endochrome; it is this which, with the primordial utricle, essentially constitutes the cell, and which furnishes the seat of all the vital phenomena exhibited by the cell. Among these phenomena is one which has not yet been adverted to, as it is not certainly known to pertain to all cells; it is the existence of a circulatory motion of the fluid contents of the cell

, a motion marked by the revolution of the floating granules, as they are carried along with the current. The motion appears to be confined to the somewhat viscid layer which immediately lines the primordial utricle. Some of the aquatic plants, especially the Characeæ, furnish fine examples of this circulation; we may see a single broad current passing up one side of the cell and down the other, and returning continuously into itself. This current is strong enough to bear along with it granular masses of starch, chlorophyll, and albuminous matter, of considerable size. The ascending and descending currents do not come into actual contact, as a clear space between them may be detected in which there is no motion. If now the cell be tied across, the current is shortly reëstablished in each portion, as if the cell had naturally divided itself. The vigor of this movement depends on the vital activity of the cell, increasing and diminishing in accordance therewith. In some cells, instead of one stream, there are several distinct currents, and these are observed to have a common point of departure and return, viz: a collection of granular matter attached to the wall of the cell, in one mass, and called the nucleus.

We have already described the mode of cell-multiplication in plants of the simplest structure ; and it holds good of the higher plants, and the highest animals also have their origin by the same mode ; and the process continues on the same plan, until certain parts of that mass begin to undergo transformation into heterogeneous structures, when it is superseded by some other method.

Cells sometimes originate in a process, termed, from its peculiarity, "budding. A certain portion of the primordial utricle seems to undergo increased nutrition ; for it is seen to

« EelmineJätka »