FERTILISATION AND EARLY DEVELOPMENT
We may now turn our attention to the consideration of some of the
phenomena connected with the early processes in the development of the
embryo. We may assume that the eggs and sperms have reached such a stage
in their life history that they are now mature. All that is necessary in
order that the development of an embryo should result is that union of
the two elements should take place. Many complicated changes have
occurred in the constitution of these eggs and sperms before this stage
is reached, but into these we need not enter. It will suffice for our
purpose to assume that they are now mature. Then as the result of a
natural instinct which suggests certain thoughts and emotions to the
male and female animals, which in turn are followed by certain definite
acts, the sperm-cell from the male and the egg or ovum-cell from the
female are brought into contact. This contact takes place in such
circumstances that the united elements are able to be protected and
nourished and so, fertilisation having thus occurred, development
begins.
The characters of these two wonderful cells, which by their union
ultimately cause the production of an embryo, are briefly as follows.
The element from the male, the sperm that is, is an extremely minute
cell which is only about 1/300 of an inch in length. As seen under a
high power of the microscope it is composed of two portions which are
spoken of as a head and the tail. The former is a flat, oval part, and
behind this is the rounded body ending in the long tail which is some
four-fifths of the total length. This long tapering tail gives to the
sperm its power of movement, for it is supposed that as the result of
the rotating or lashing movements of this tail the cell is propelled.
Indeed its rate of motion has been actually studied, and estimated to be
at about one-eighth of an inch per minute.
The cell contributed by the female, the ovum that is, has quite a
different structure and microscopical appearance. Compared with most
cells it is rather large, almost round in shape, having a diameter of
about 1/120 of an inch. Up to the time we are now considering, this
cell, along with a great many others like it, has been stored within the
female ovary, from which organ an ovum periodically escapes. Unless
fertilisation takes place by union with a sperm the discharged ovum
perishes. Should, however, the sperm-cell be available, and should it
have been able to reach a situation at which fertilisation can take
place within, the chain of events which constitute development begins.
But before fertilisation can take place the ovum has undergone what is
called the process of maturation, in which it divides twice, giving off
two small portions of itself in the process. The result of this is that
half the number of chromosomes in the ovum are lost. This process of
maturation has already taken place in the sperm before it leaves the
body of the male.
When these two cells meet, the actual fusion of their material takes
place, the head of the sperm penetrating into the substance of the ovum,
and the body of the sperm completely fusing with the nucleus of the
ovum. This gives rise to what is called the "segmentation nucleus." It
will be observed that we now have a cell in which the full number of
chromosomes for that particular species is represented once more. But
this full number has now been made up from two different sources, half
from the elements contributed from the male, and half from those of the
female. It is at this stage that the inherited tendencies, carried in
the germ-plasm on the two sides of the ancestry, become mingled, and
from thenceforward the division of the fertilised cell into many
cell-descendants goes on with extreme rapidity.
Two different lines of germ-plasm have thus been intimately mingled, and
the actual significance of this mingling has given rise to one of the
most acutely debated points in all the problems of heredity. Put into
quite plain language that problem is--What is the function of sex? It is
no part of our task here to answer that problem, but it is of interest
to point out precisely at what stage it occurs in embryology. The
obvious answer, however, may be advanced that the function of sex is to
mix the characters of the parents in such a way that some from each
source will be found in the offspring. But how these are mixed, whether
as painters mix two colours and produce a third, or as two packs of
cards are mixed having different coloured backs, is quite another
matter.
The fertilised ovum now commences to form a number of successive
generations of cells, and this it does by dividing into two, four,
eight, sixteen, thirty-two, and so forth, until a number of cells have
been produced which arrange themselves into the form of a ball. The
surface of this ball resembles that of a mulberry, each elevation
corresponding to a cell. This mass is termed by embryologists "the
morula." (See Fig. 1.)
[Illustration: FIG. 1.]
Next, within this morula some of the cells become condensed into one
particular portion, leaving a space which contains fluid. The ball is
now no longer solid, but has a portion consisting of cells, and a
portion consisting of fluid. It is now called a "blastocyst." (See Fig.
2.)
[Illustration: FIG. 2.]
The cross-section of this shows the cells projecting into a cavity. This
is the first attempt of the fertilised ovum to form itself into the
different layers, which are ultimately going to give rise to all the
different tissues of the embryo. But it is interesting to know at this
stage that the outer layer of cells, those representing a margin in the
figure, has nothing to do with the forming of the embryo at all, but
gives rise to a structure whose function afterwards is to be that of
nourishing the growing embryo.
The next obvious change is that the cells at the lower portion of the
mass which projects into the cavity appear to get flattened out--at any
rate they obviously arrange themselves in a definite and separate layer;
and this layer in its turn proceeds to go on growing by division of its
cells in such a way as to form another little closed cavity within the
larger one. This cavity is termed the "yolk sac." (See Fig. 3.) Then
another little cavity occurs, this time within the original projecting
cell-mass. This cavity is termed by embryologists the "amniotic cavity,"
and the cells which line it, and which in their turn become arranged as
a separate layer, form what is termed the "embryonic ectoderm." (See
Fig. 3.)
[Illustration: FIG. 3.]
It is in this region, and in that of the yolk sac which lies just
underneath it, that the future growth of the embryo itself occurs, and
the portion is therefore termed the "embryonic area." (See Fig. 3.)
Up to this point we have seen that two layers of cells have appeared,
one round the yolk sac, called the "entoderm," and the other lining the
amnion, called the "ectoderm." After these two germinal layers have made
their appearance, a third layer comes into existence, which, because it
begins growing from the embryonic area, and lies between the two already
mentioned, has received the name of the "mesoderm." This third germinal
layer divides into two portions before very long, and the space between
these two is that in which the body cavity itself subsequently arises.
One part of the mesoderm, situated near one end of the embryonic area,
is specially important, because in it are formed the blood-vessels which
supply the embryo, and which ultimately afterwards becomes the
"umbilical cord," which forms the connection between embryo and mother.