A cell is a living particle, so minute that it needs a microscope for
its examination; it grows in size, maintains itself in a state of
activity, responds to the action of stimuli, reproduces its kind and in
the course of time it degenerates and dies.
Let us glance at the structure of a cell to determine its constituent
parts and the rôle which each plays in the function to be discharged.
The original conception of a cell, based upon the study of the
vegetable tissues, was a minute vesicle inclosed by a definite wall,
which exercised chemical or metabolic changes on the surrounding
material and secreted into the vesicle its characteristic contents. A
similar conception was at first also entertained regarding the cells
of animal tissues; but as observations multiplied, it was seen that
numerous elementary particles, which were obviously in their nature
cells, did not possess an inclosing envelope. A wall ceased to have a
primary value as a constituent part of a cell, the necessary vesicular
character of which therefore could no longer be entertained.
The other constituent parts of a cell are the cell plasm, which forms
the body of the cell, and the nucleus embedded in its substance.
Notwithstanding the very minute size of the nucleus, which even in
the largest cells is not more than one-five-hundredth of an inch in
diameter, and usually is considerably smaller, its almost constant
form, its well-defined sharp outline and its power of resisting the
action of strong reagents when applied to the cell, have from the
period of its discovery by Robert Brown caused histologists to bestow
on it much attention. Its structure and chemical composition; its mode
of origin; the part which it plays in the formation of new cells, and
its function in nutrition and secretion have been investigated.
When examined under favorable conditions in its passive or resting
state, the nucleus is seen to be bounded by a membrane which separates
it from the cell plasm and gives it the characteristic sharp contour.
It contains an apparently structureless nuclear substance, nucleoplasm
or enchylema, in which are embedded one or more extremely minute
particles called nucleoli, along with a network of exceedingly fine
threads or fibers, which in the active living cell play an essential
part in the production of new nuclei within the cell. In its chemical
composition the nuclear substance consists of albuminous plastin and
globulin; and of a special material named nuclein, rich in phosphorus
and with an acid reaction. The delicate network within the nucleus
consists apparently of the nuclein, a substance which stains with
carmine and other dyes, a property which enables the changes, which
take place in the network in the production of young cells, to be more
readily seen and followed out by the observer.
The mode of origin of the nucleus and the part which it plays in the
production of new cells have been the subject of much discussion.
Schleiden, whose observations, published in 1838, were made on the
cells of plants, believed that within the cell a nucleolus first
appeared, and that around it molecules aggregated to form the nucleus.
Schwann again, whose observations were mostly made on the cells of
animals, considered that an amorphous material existed in organized
bodies, which he called cytoblastema. It formed the contents of cells,
or it might be situated free or external to them. He figuratively
compared it to a mother liquor in which crystals are formed. Either in
the cytoblastema within the cells or in that situated external to them,
the aggregation of molecules around a nucleolus to form a nucleus might
occur, and, when once the nucleus had been formed, in its turn it would
serve as a center of aggregation of additional molecules from which
a new cell would be produced. He regarded, therefore, the formation
of nuclei and cells as possible in two ways--one within preëxisting
cells (endogenous cell-formation), the other in a free blastema lying
external to cells (free cell-formation). In animals, he says, the
endogenous method is rare, and the customary origin is in an external
blastema. Both Schleiden and Schwann considered that after the cell was
formed the nucleus had no permanent influence on the life of the cell,
and usually disappeared.
Under the teaching principally of Henle, the famous Professor of
Anatomy in Göttingen, the conception of the free formation of nuclei
and cells in a more or less fluid blastema, by an aggregation of
elementary granules and molecules, obtained so much credence,
especially amongst those who were engaged in the study of pathological
processes, that the origin of cells within preëxisting cells was to
a large extent lost sight of. That a parent cell was requisite for
the production of new cells seemed to many investigators to be no
longer needed. Without doubt this conception of free cell-formation
contributed in no small degree to the belief, entertained by various
observers, that the simplest plants and animals might arise, without
preëxisting parents, in organic fluids destitute of life, by a process
of spontaneous generation; a belief which prevailed in many minds
almost to the present day. If, as has been stated, the doctrine of
abiogenesis cannot be experimentally refuted, on the other hand it has
not been experimentally proved. The burden of proof lies with those who
hold the doctrine, and the evidence that we possess is all the other
way.