Of the organs of secretion. — In order that we rnay duly understand the real nature of the secreting process, as elucidated by recent discoveries, it is requisite that we should ex amine into the nature of the instruments by which it is effected. There can scarcely be a more beautiful illustration of the doctrine that physiology is as capable as any other science of being reduced to general prin ciples, and that these principles must, if valid, be of universal operation, than the fact that the process of secretion—common as it is in all its essential features to the animal and vegetable kingdoms—is every where per formed by the same agency, namely, the de velopment of simple cells, each possessing its own independent vitality ; these bodies form ing the really operative part of every secreting organ, however complex its structure may be. The prooress of comparative anatomy has shown tilt neither the form nor the in ternal arrangement of the parts of a gland could have any essential connection with the nature of its product (see GLAND) ; since even those glands (the liver and the kidney, for example) in which there is the greatest complexity of structure, make their first appearance at the lower end of the animal series, as in the early embryo of the very highest, in the simplest possible form. Still something was wanting to prove that the structural elements immediately concerned are in all instances the same ; and there seemed no analogy whatever between the secreting membrane of the animal and the secreting cell of the plant. The doctrine was first propounded by Purkinje 4' and Schwann t, adopted and extended by Ilenle and fully confirmed by the researches of Goodsir § and Bovvman that the true pro cess of secretion — under whatever form it may present itself—is always performed by the intervention of cells ; which, as part of their own regular vital actions, select and withdraw certain ingredients from the nu tritive fluids, and afterwards set them free again, generally by the rupture or dissolution of the cell-wall, but sometimes perhaps by a simple act of transudation. Po: the proper comprehension of this doctrine in all its ge nerality, it is necessary to give some attention to the history of cell-development, as mani fested in the simplest forms of organic ex istence ; those cryptogamic plants, namely, in which every cell is a distinct and inde pendent individual.
The earliest condition of such a cell is a minute molecule, which cannot be discerned except under a considerable magnifying power, and in which even the highest ampli fication fails to exhibit any distinction of parts. When placed under circumstances favourable to its development, namely, when supplied with the materials of its nu trition, and stimulated by the requisite de gree of warmth, —this germ increases in size; and a distinction becoines apparent between its transparent exterior and its coloured interior. Thus we have the first indication between the cell-wall and the cell-cavity. As the en largement proceeds, the distinction becomes more obvious ; the cell-wall is seen to be of extreme tenuity and perfectly transparent, and to be homogeneous in its texture, whilst the contents of the cavity are distinguishable in the Algm by their colour, which is green in the Chlorococcz, and bright red in the Hxma tococci ; but in the simple fungi, such as the Torula cerevisii, or yeast plant, they are colourless. The contents of the cell-cavity have no relation whatever to the material of the cell.wall. Of this we have a remarkable example in the cases just cited ; for whilst the red and green coloured products of the Algm are probably nearly related to each other and to the chlorophyll of higher plants, being simple ternary compounds of water and carbon, the cell-contents of the yeast-plant are closely allied to the protein compounds ; and yet the cell-walls in both instances are composed of the same material, cellulose. It is evident, then, that the inherent powers of the cell are not confined to the application of nutrient materials to the extension of its own walls, and the consequent enlargement of its cavity ; but that they are exercised also in selecting from (and it may be in combining or modifying) the same materials, in order to fill this cavity with a certain product, which inay be altogether different in its constitution and its properties from that of which its wall is composed. This latter process is as essential
to our idea of a living cell, as is the growth of its wall; and must never be left out of view when the history of cell-development is being considered.
The nature of the compound thus stored up in the interior of a cell depends in part upon the original inherent endowments of the cell itself, derived from its germ ; and, in part, upon the character of the nutriment supplied to it. Thus we find that the simple Alge will grow wherever they can obtain, from the air and moisture around, the elements of their cell-walls and of their cell-contents ; vvhich elements they have themselves the power of combining into those peculiar com pounds, of which analysis shows that they are composed. But out of the very same materials, and under circumstances to all appearance identical, the Chlorococcus manu factures a green product, and the Ilmmato coccus a red one. On the other hand the yeast-plant, like the fungi in general, will only grow where it meets with an azotised compound already formed ; and from this it elaborates the product which occupies its cell-cavity, its cell-wall being apparently formed by the same process as that of the simplest Algm. It could no more vege tate, as they do, upon cold damp surfaces, than they could develop themselves in a solution of fermentible matter secluded from the light.
A similar variety of function is seen amongst the cells, whose aggregation makes up the structure of any one of the higher plants, and which are all the descendants of the single cell which constituted its original germ. Thus we have in the green cells of the leaves the representatives of the simple Chlorococci ; these, under the influence of solar light, com bining the carbon which they derive from the atmosphere, or from the soil, with the water transmitted from the roots, and elaborating these elements into a variety of new products, amongst which chlorophyll and cellulose are still prominent ; but also operating upon the azote which they draw from the atmosphere or from the soil, and combining this with the other three elements into quaternary com pounds, that seem destined rather for the nutrition of animals than for any special pur pose in the economy of the plant itself. The contents of the cells of the leaves are thus of a very coinplex nature ; their life not be ginning and ending with themselves, as is the case with that of the independent organisms, which in other respects they resemble ; but having relations to the rest of the structure, for which, in fact, it is their function to pre pare the pabulum. For the elaborated sap or nutritious fluid, which is the product of their agency, is transmitted through the entire fabric, and furnishes each portion with the materials of its development and extension, which in every instance is effected by an act of cell-growth. All parts select from it the same substance for the formation of the cell walls, but the cell-contents are different in every organ and variety of tissue. Thus we find one set of cells drawing in starch, another fixed oil, another resin, another volatile oil, another colouring matter, another sclerogen, another protein compounds, and so on ; and this with the greatest uniformity and regu larity'. We may frequently see that even contiguous, and in other respects similar, cells, in the same organ, either select from the common pabulum a different compound, or exercise upon the same compound a dif ferent influence. Thus we observe in the parti coloured petal of a hearts-ease or tulip, certain stripes or patches of different hues, which, when examined with the microscope, are found to consist of cells that differ from each other only in the colour of their contents. A precisely similar phenomenon is presented by the epidermic cells, which constitute- the scales of the wings of Lepidoptera.