PROBLEMS OF REPRODUCTION
We have seen that in the higher types of animals and plants the single
individual is made up of not one but millions and millions of cells
united together for the common purpose of the individual life, and that
in such complicated individualities some cells perform one function
while others perform others. A human individual from this point of view,
therefore, is an organised community of cells all of which, however,
sprang, in the first place, from one single cell. That original single
cell is termed, in animal Embryology, the "fertilised ovum." It is
popularly spoken of frequently as "the egg." All the other millions of
cells are the direct descendants of this fertilised ovum, or egg, even
though many of them eventually become extremely unlike the original
cell. In single-celled animals the offspring of the original cell remain
like the parent cell, but in the highly complicated creatures the
offspring split up into a great many types of cells, owing to the very
fact that all remain adherent together to form the mass of the body in
order to carry out different functions. So we find cells of one type in
glands, of another type in the brain, of another type in bones, of
another type in blood, and so forth. Nevertheless all of them sprang
from one original single cell. None of these specialised types of
cells, however, are capable of performing any other function than their
own. A bone-cell cannot receive an impression, nor originate an idea,
any more than a brain-cell can secrete bile. Each kind of cell has its
own appointed duty. The most important duty that can possibly be
allotted to any cell is obviously that of reproducing the individual for
the purpose of continuing the race or species. So we find in higher
animals that this function, like others, is relegated to a special set
of cells also derived from the original single cell, and which are
called "the germ-cells."
Leaving out of consideration the question of reproduction in lower types
of animals we may consider the nature and origin of these cells in
highest vertebrates, such as the mammals, including man. Germ-cells,
which are derived from the tissues of a female animal, are termed "ova."
Those which are derived from the tissues of a male animal are termed
"sperms." Notice that it is not these germ-cells themselves to which the
terms male and female, indicative of the two sexes, are applied, but
only to the individuals. They are male and female; the germ-cells are of
neither sex. True the germ-cells from the male, _i.e._ the sperms,
differ in appearance when seen under the microscope from those of the
female, but there is no reason to believe that there is any difference
between them in their capacity, for example, of transmitting the
characters of ancestors to succeeding generations.
At a certain stage in the life history of the animal individual and
after undergoing certain changes which need not be considered here,
these germ-cells, both sperms and ova, have reached such a stage of
maturity as to be capable of carrying on their sole function, namely,
that of reproducing the species. The actual age in human beings, for
example, at which this maturity is reached varies very much in different
races, and in different individuals of the same race.
When reproduction is about to occur a union must take place between a
germ-cell from a male body with a germ-cell from a female body; that is
to say, a union must take place between a sperm and an ovum. This union
takes place within the body of the female individual and results in the
fusion of the two cells into one single cell, which is now termed a
fertilised ovum. This fertilised ovum, in virtue of this process of
union, is now able under suitable conditions of nutrition and shelter,
such as it obtains within the female organs of reproduction, to divide
and redivide again and again, thus building up a new mass of cells as
the result of its division. The millions of cells so produced include,
as we have already seen, cells which have all the various functions
which are necessary for the continuation of the life of a human
individual; that is to say, that as the result of this division of the
fertilised ovum there are produced first of all germ-cells to secure the
still further continuance of the race, and then multitudes of all the
other kinds of cells which gradually assume the shape of an embryo or
young individual, and ultimately grow into a human being.
In all the highly complicated animals fertilisation by union of
germ-cells from male and female must precede reproduction. The result
eventually is this multicellular individual composed of a number of
different kinds of cells each set apart for its own work. But it is well
to recognise that we may regard all these cells as really of two kinds,
namely, the germ-cells and the others. That is to say, two kinds of
cells are produced as the result of fertilisation, namely, cells whose
business it ultimately will be to again take part in a similar process
of fertilisation, and so perpetuate the species, and all the other cells
which go to the forming of the various body tissues of the individual
itself. In this way we get a simple classification of the cells which
form, for example, a human being, namely, germ-cells and body-cells, the
latter often being termed "somatic." The latter are, of course, in much
greater abundance than the germ-cells. They have to form all the various
elements, organs, limbs, and structures known and described by the
anatomist. The germ-cells are a separate little group of themselves
embedded in the male and female reproductive organs for the sake of
nutrition, growth, and shelter, for many years, until they again take
part in the process of fertilisation. Note carefully that no other cells
in the body ever unite together to produce a new individual except
germ-cells.
Somatic cells reproduce by dividing directly. Germ-cells before they can
do this require to be fertilised. That is to say, the cell from the male
(the sperm) must fuse with the cell in the female (the ovum). As Dr.
Archdall Reid graphically states it, "Only the germs are marriageable;
and, as we have just seen, in the great majority of animals and plants
they observe the degrees of consanguinity very strictly, and do not
unite except with members of another cell-community, and then only to
found a new colony of cells, an offspring."
There are still some further considerations in connection with the
subject of germ-cells and germ-plasm which we must carefully consider
before leaving this part of our subject, Embryology. Everything depends
upon a perfectly clear understanding at this stage. The facts themselves
that have to be adduced in this connection are comparatively few and
simple. No fairly educated person should have any difficulty whatsoever
in grasping them. Moreover, very fortunately they are thoroughly well
established and not in dispute. But the reasoning which is based upon
these few and elementary facts, reasoning which is applied to the
methods of treatment of the individual which is produced, may be very
complicated and very debatable. Various schools of thought and opinion
exist according to the attitude taken towards the facts, some of which
we have mentioned and others of which we are about to detail. But the
facts themselves are not debatable, and we therefore see once more that
their importance at this stage cannot be exaggerated.
One or two very simple general propositions bound up with the subject of
Embryology, or individual development, may be stated in order to focus
attention upon the nature of the problem under investigation. Thus
nobody will be found to question the fundamental truth that children
resemble their parents. That is a commonplace of experience. Similarly
no one will be found to dispute another fundamental fact, namely, that
children differ from their parents. This, too, is equally a commonplace
of experience. If we examine a million human beings we find that they
all possess certain features in common, certain characteristics in
virtue of which we recognize them to be human beings. Nevertheless it is
just as true that a careful examination of the same million people
reveals the true saying that no two of them are exactly alike. Here then
are two propositions equally true within certain limits; namely, that
all human beings resemble each other, and that all human beings differ
from each other. There is resemblance; and there is variation. These two
things are universal because of the existence and characteristics of
germ-cells. We may look at this a little closer.
Every species of animal, in the process of reproduction brings forth
offspring similar to itself. This is expressed in the familiar proverb
that "like produces like." One does not expect grapes from thorns, nor
is it possible to construct a silk purse out of a sow's ear. But what is
the explanation of this proverbial fact? The answer is of great
importance, because although the fact itself is recognized as a general
principle in the reproduction of a species, it is not sufficiently
recognized in the full details of the characters of that individual. Too
many people are still apt to expect to be able to produce grapes when
the plant is a thorn, and it is unfortunately all too common to make
heroic but quite futile attempts to construct human silk purses out of
human sows' ears--so to speak--simply because of the ignorance of the
material which is being used. The most that can be done is to give such
material as is present the very best opportunity of attaining its own
utmost perfection; and this, by the way, is vastly more than has ever
been done for any considerable number of the human race.
But why this continuity of species? Why should like always produce like?
The answer has been sought by biologists ever since problems of life
attracted man's curiosity. All sorts of weird and fantastic theories
have been put forward at different times to account for this simple
fact, but it is only in comparatively recent years that the real
explanation has been forthcoming. It is perfectly obvious that in order
to secure this continuity of racial resemblance there must be something
physical or material which is actually continuous from generation to
generation to account for it. The immortal Darwin saw this very clearly,
and devoted much thought in the endeavor to find some explanation of
this very problem. The result was his theory of Pangenesis which,
ingenious as it was, was ultimately shown to have no basis on fact. In
his effort to account for the fact that children resemble their parents
even in such minute details as the shape of the nose, the colour of the
eyes, and so forth, he formulated the idea that the parents themselves
probably contributed multitudes of minute particles from their own
tissues to form the cells of their offspring. He supposed, for example,
that particles or gemmules from the eyes, nose, hair, and so forth, of
the parent, or parents, in some way or other were fused together and
gave rise to the cells which ultimately produced an embryo. Hence he
thought the explanation of the resemblance between parents and children.
This was his solution to the question of the physical continuity between
successive generations. It may be remarked in passing that it is with
something of pathos that one reads in Darwin's own works his own evident
opinion that this theory of Pangenesis was a great discovery. One
gathers almost that he himself regarded it as of greater importance than
his work on natural selection.
In the course of time, however, the real actual basis of physical
continuity was shown to be something quite different, and looking back
now upon the history of the discoveries in this connection during the
last generation one can easily imagine what speculations there must have
been in the absence of the facts which are now known to embryologists.