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Updated: May 23, 2025
These are associated and combined in many ways. They are arranged in distinct masses called nerve centers, or in the form of cords known as nerves. The former are made up of nerve fibers; the latter of both cells and fibers. Nerve cells, which may be regarded as the central organs of the nerve fibers, consist of masses of cell protoplasm, with a large nucleus and nucleolus.
In brief, then, a cell is a mass of nucleated protoplasm; the nucleus may have a nucleolus, and the cell may be limited by a cell wall. Every tissue of the human body is formed through the agency of protoplasmic cells, although in most cases the changes they undergo are so great that little evidence remains of their existence.
Like other cells it has a cell wall, a cell substance with its linin and fluid portions, a nucleus surrounded by a membrane and containing a reticulum, a nucleolus and chromatic material, and lastly, a centrosome.
This resemblance is so close in the main features that we have only to repeat what we have already said of the ontogenesis of the Amphioxus. In the thick fine-grained yelk we find a clear round germinal vesicle of about 1/750 of an inch in diameter, and this encloses a small embryonic spot or nucleolus.
If the body be studied in its development, it is found to originate from a single mass of nucleated protoplasm, a single cell with a nucleus and nucleolus. From this original cell, by growth and development, the body, with all its various tissues, is built up. Many fully formed organs, like the liver, consist chiefly of cells.
A great enlargement in one direction of several oval pipes would give rise to the lamellae. Besides the free calciferous cells in which no nucleus was visible, other and rather larger free cells were seen on three occasions; and these contained a distinct nucleus and nucleolus. They were only so far acted on by acetic acid that the nucleus was thus rendered more distinct.
With the microscope at his eye, he magnifies nature's mysteries; he sums up the investigations of the Hertwig brothers; he discourses learnedly of the nucleolus of the Cytula or progeny cell. He declares that science is able to watch the creation of a human being, as it watches the progress of a chick in the egg.
One organisation is made up of other organisations—cells, protoplasm, nucleus, nucleolus, centrosomes, and so on, according to the power of the microscope; and these structures, instead of explaining the vital functions of growth, development, multiplication by division, and the rest, simply repeat them on a smaller scale, and are thus in their turn living units, the aggregation of which is illustrated better by the analogy of a social organism than by that of a mechanical structure.
My horizon of scientific possibilities has suddenly been enormously extended. I no longer wonder at Wilson's demonic energy and enthusiasm. Who would not work hard who had a vast virgin field ready to his hand? Why, I have known the novel shape of a nucleolus, or a trifling peculiarity of striped muscular fibre seen under a 300-diameter lens, fill me with exultation.
Now, while there are found in the lowest realm of animal life, organisms like the amoeba of stagnant pools, consisting of nothing more than minute masses of protoplasm, there are others like them which possess a small central body called a nucleus. This is known as nucleated protoplasm. A, nucleus; B, nucleolus; C, protoplasm. Cells.
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