active substance of vegetable cells. This term was extended by Max Schultze to embrace all organic cells, and he defined the cell as a nucleated mass of protoplasm. At a still later period it was declared that a nucleus was not always present, and the cell was defined as "a structureless mass of protoplasm."
Such was the stage of the cell-doctrine reached in 1872, thirteen years ago. First the cell-wall had been considered the active element, then the nucleus, and finally the protoplasmic contents, while wall and nucleus came to be considered inessential elements. As Drysdale expressed it about that date, "a cell is like a gun-barrel, without a stock and a lock." Meanwhile Beale had persistently declared that there is no such thing as a cell, in the ordinary sense of the term; but that all organic bodies are made up of minute particles of living or germinal matter, which consume nutriment and increase internally, while their exterior portions lose vital activity, and become dead or formed material. These living particles not only grow, but divide, and thus set up new centers of growth, from which emanates new-formed material.
The division of the cell-protoplasm is, indeed, a most essential part of the life-process, and to it growth and differentiation of tissue are principally due. The cell, when furnished with nutriment, manifests individual growth for a short period. Then it separates into two or more new cells, each of which sets up an individual life. This separation takes place in several methods, of which the most common is by an equatorial constriction, which gradually deepens until it cuts the cell into two sections. Other methods are by the budding off of minute portions from the surface, or the transformation of the cell-contents into many minute germs, which are subsequently set free.
Such was the cell of thirteen years ago—"a structureless mass of protoplasm," which increased in size by nutrition, and in numbers by division. Such is the cell of most of the text-books of to-day. But the cell of science is a very different affair. Instead of being structureless, it is found to possess an intricate structure, while its division is far from being the simple process above indicated. The new cell-theory is, in fact, but five or six years old in its developed form, and it is as yet settled only in its main features. Its minor details need much further elucidation.
These new discoveries, which we shall briefly describe, are largely due to the increased power and clearness of definition of the microscope, and still more to new and improved methods of preparing organic sections for investigation, by the employment of stains, preserving agents, and other useful appliances. It is not every microscopist that is able to see the minute details of cell-structure lately announced. The careful preparation of material and exceedingly delicate manipulation required need years of practice, and the discoveries referred to are due to the first microscopists of the age, though the methods are now so
