We have now to retrace our steps and look somewhat more closely into the discovery of that important body, the cell-nucleus. It was an English botanist, Dr. Robert Brown, who, in 1833, during his microscopical studies of the epidermis of orchids, discovered in their cells “an opaque spot,” to which soon afterwards he gave the name of nucleus. Schleiden and Schwann’s later researches led them to the conclusion that the nucleus is the most characteristic formative element in all vegetable and animal tissues in the incipient phase of existence. It then began to be taught that there is one universal principle of development for the elementary parts of all organisms, however different, and that is the formation of cells. Thus was enunciated a doctrine which was for all practical purposes absolutely new, and which opened out a wide field of further investigation for the physiologist, and led up to a fuller knowledge of the cell contents. In fact, it became a question as to whether the cell contents rather than the enclosing wall should not be considered the basis of life, since the cell at this time had by no means lost its importance, although it no longer signified the minute cavity it did when originally discovered by Schwann. It now implied, as Schultz defined it, “a small mass of viscid matter, protoplasm, endowed with the attributes of life.” The nucleus was once more restored to its original importance, and with even greater significance. In place of being a structure generated de novo from non-cellular substance, and disappearing as soon as its function of cell formation is accomplished, the nucleus is now known as the central permanent feature of every cell, and indestructible while the cell lives, and the parent, by division of its substance, of other generations of nuclei and cells. The word cell has at the same time received its final definition as “a small mass of protoplasm supplied with a nucleus.” In short, all the activities of plant and animal life are really the product of energy liberated solely through histolysis, or destructive processes, amounting to the combustion that takes place in the ultimate cells of the organisms.
But there are other points of especial interest involved in the question of cell formation beside those already mentioned.
The cell and its contents collectively are termed the endoplasm, or when coloured, as in algæ, endochrome. With regard to the outer layer of the cell and its growth nothing satisfactory has been clearly determined and finally accepted.
The cell as a whole is a protoplasmic mass, and not an emulsion, as some observers would have us suppose. It is, in fact, a reticulated tissue of the most delicate structure, made up of canaliculate spiral fibrils with hyaline walls capable of expansion and contraction. These fibrils are probably composed of still finer spirals. The visible granulated portion of the protoplasm, the only part that takes a stain under ordinary circumstances, is simply the contents of these canals. It is the chromatin of Flemming, and is capable of motion within the canals. The nucleus, then, is probably nothing more than a granule of the extra-cellular net, and is formed by the junction of the several bands of wall-threads which traverse it in different directions. The cell wall of plants possesses the same structure as protoplasm, and is probably protoplasm impregnated by cellulose.
It is this portion of the protoplasmic mass that is now recognised under the term octoplasm, or primordial utricle, and is of so fine and delicate a nature that it is only brought into view when separated from the cell wall either by further developmental changes, or by reagents and certain stains or dyes. It was, in fact, discovered to be a slightly condensed portion of the protoplasmic layer corresponding to the octosare of the lower forms of animal life. The octoplasm and cell wall can only be distinguished from each other by chemical tests. Both nucleus and nucleoli are only rendered visible in the same way, that is, by staining for several hours in a carmine solution, and washing in a weak acetic acid solution.
With the enlargement of the cell by the imbibition of water, clear spaces, termed vacuoles, are seen to occupy a small portion of the cell, while the nucleus and nucleoli lie close to the parietal layer.
The interesting phenomenon of cyclosis, to which I shall have occasion to refer further on, is now believed to be due to the contractility of certain wall-threads stretching from the nucleus to the outermost layers of the cell. An intimate relationship is thereby established between the nucleus, the nucleolus, and the parietal layer. This much has been made clear by the more scientific methods of investigation pursued in the use of the microscope. Nevertheless a large and important class of cells, forming a kind of borderland between the vegetable and animal kingdoms, still remains to be dealt with, in which the cell contents are only imperfectly differentiated, while numerous other unicellular organisms, owing to their extreme minuteness, tenuity, and want of all colour, are apparently devoid of any nucleus, and when present can only be differentiated by a resort to a specially conducted method of preparation and staining. There is, however, a remarkable feature in connection with many micro-organisms—that certain of these protophytes possess motive organs, cilia or flagella, bodies at one time supposed to be characteristic of, and belonging to, the protozoa.
This being the case, the methods of plant chemistry are of supreme interest, the more so because physiologists are in a position to isolate a single bacterial cell, grow it in certain media, and thus devote special attention to it, and keep it for some time under observation. In this way it has become possible to further grasp facts in connection with cell nutrition and the nature of its waste products. We have, then, arrived at a stage when the history of the chemical changes brought about by bacteria can be more definitely determined, as we have here to do with the vegetable cell in its simplest form. The chemical work performed by these micro-organisms has as yet occupied only a few years; nevertheless, the results have been of the most remarkable and encouraging character.
At an earlier period an interesting discovery in connection with the pathogenic action of these bodies was, by the labours of Schöenlein, Robin, and others, brought to the notice of the medical profession, viz., that certain diseases affecting the human body were due to vegetable parasites. In 1856 an opportunity offered itself for a thorough investigation, and the microscopical part of the work fell into my hands, with the result that I was able to add considerably to Schöenlein’s list of parasitic skin diseases. My observations were in the first instance communicated to the medical journals. But the generalisation arrived at was that “If there be any exceptions to the law that parasites select for their sustenance the subjects of debility and decay, such exceptions are rarely to be found among the vegetations belonging to fungi, which invariably derive nutrition from matter in a state of lowered vitality, passing into degeneration, or wherein decomposition has already taken place to a certain extent.... It scarcely admits of a doubt that all diseases observed of late years among plants have been due to parasites of the same class favoured by want of vigour of growth and atmospheric conditions, and that the cause of the various murrains of which so much has been heard is also due to similar causes.”[50]
Herein, then, is to be found the solution of a difficulty that so long surrounded the question, but which subsequently culminated in the specialisation and scientific development of bacteriology, due to the unceasing labours of Pasteur, whose solid genius enabled him to overcome the prejudices of those who were at work on other lines, and who had no conception of the functions that parasitic organisms fulfil in nature.