The nucleolus is round, sharply defined, and often so small as to be almost immeas urable. Nucleoli are found in most nuclei so long as the latter are still young, and in many during their whole existence. 1s, however nuclei exist in which no nucleolus can be detected, we cannot regard the nucleolus as so essential an element of the cell as the nucleus. Most commonly a nucleus contains only one nucleolus; two are not unfre quently seen; more are rare.
Our knowledge of the chemical composition of C. is very imperfect. That the cell membrane is a protein substance (q.v.)—at all events in young C.—is obvious from its solubility in acetic acid and in dilute caustic alkalies; and the membrane of the necleus seems to have a similar composition; while there are chemical reasons for believing that the nucleolus is composed of fat. In the contents of most C. we usually find such sub stances as occur in solution in the cytoblastema—viz., water, albumen, fat, extractive matters, and salts; and in the C. of secreting organs, as for instance, the liver and kid neys, we find the special secretions of those glands; in the blood-cells, we find hema tocrystalline, etc.
There are two perfectly distinct ways in which C. can be generated: they may be developed independently of other C. in a plastic fluid (the cytoblastema); or they may be developed from pre-existing C. by cell-multiplication, the existing C. either produc ing secondary C. within themselves, or multiplying by division. In both these latter kinds of cell-development, the nucleus seems to be the center of development of the young cells.
In order that free or independent cell-development shall take place, we must have a cytoblastema containing protein substances (probably fibrin), fat, and certain salts (especially phosphates) in solution; and very possibly the presence of the particles of pre-existing C. may also be necessary, in which case free cell-development ceases to exist. The chyle and lymph corpuscles may be mentioned as examples of this mode of cell-formation. The steps of the process are not very clearly made out, but we know that the nuclei are first formed, and that the cell-membranes are developed around them. Free cell-development is far less common in man and the higher animals than cell-mul tiplication, and, we believe, never occurs in the vegetable kingdom. All pathological cell-formations--the C. in pus (q.v.), and in other morbid exudations—come, however,
under this head.
The development of C. within other C. is of very common occurrence. An original or parent cell produces two or more secondary or daughter C., and the process of forma tion is said to be endogenous. Cartilage-cells afford a good example of this process. The nucleus and the contents of each parent cell undergo division into two parts, so that the number of C. is successively doubled. The mode in which the multiplication of the nucleus takes place cannot be definitely made out in all cases, but when clear observation is possible, the nucleoli first divide into two, and then separate.
A. multiplication of C. by division has been proved to take place in the red blood-cells of the embryos of birds and mammals, and in the first colorless blood-cell of the tad pole, and very probably occurs extensively in many embryonic and adult tissues, in which a self-multiplication of C. is certain, but where no parent C. with secondary C. can be detected. In this and similar cases we have an elongation of the cell, and the single nucleus becomes divided into two; the cell then suffers constriction in the mid dle, which proceeds till it finally separates into two parts, each of which contains a nucleus. This variety of cell-formation affords a good illustration of the doubt and difficulty connected with this class of investigations. It was altogether unknown to Schwann when he published his great work in 1839, and was first noticed and described by Remak in 1841, who, however, subsequently retracted his published view, and did not again advocate it till K011iker confirmed his observation, and declared it to be correct.
No satisfactory theory has been propounded with the view of explaining the develop ment of cells. Schwaqn compares the formation of C. with that of crystals, but it must be recollected that the molecular attraction concerned in the formation of C. is so far peculiar, that-1. It never produces geometrical solids, but even in the nucleus and nucleolus determines a globular form; 2. That it aggregates not homogeneous, but chemically different substances; and 3. That the final result of its action—namely, the cell—is extremely limited in size, while a crystal may be of a comparatively indefinite magnitude.