EARLY DEVELOPMENT
The early development of the embryo now proceeds rapidly, and its
appearance at the stage we have just been describing is thus stated by
Dr. R. W. Johnstone:--
"If the ovum at this stage be looked at from above, the embryonic area
appears as a small shaded oval. The shading is due to an increased
growth of cells, because here the three germinal layers--embryonic
ectoderm, mesoderm, and entoderm--are in contact. At one end a patch of
darker shading indicates a still greater growth of cells. Running
forward from this is a band--the _primitive streak_--in the centre of
which lies a darker line--the _primitive groove_. At the far (anterior)
end of the primitive groove there is a dark spot--Hensen's node--from
which still another streak runs forward, the head process. Later, in
front of the primitive streak, a thickened band of ectoderm appears,
broadening out posteriorly. The edges of this band rise up to form two
folds, which meet anteriorly. The groove between them is the _medullary
groove_, and ultimately they fold over and unite to form the _neural
canal_. (See Fig. 4.)
"Along the line of the primitive streak all three germinal layers are in
contact. Superficial to it is the amnion, and below it is the yolk sac.
The embryonic area is the only part of the ovum which has to do with the
subsequent development of the embryo; the other parts of the
blastodermic vesicle become subservient as nutritive or supporting
structures.
"At this stage, and for the first three weeks of its existence, the
embryo is a 'flat disc floating on the surface of the yolk sac.'
(M'Murrich.)"
[Illustration: FIG. 4.]
This is followed by a folding of the embryo, due to the enlarging of the
amniotic cavity, the result being to form what may be termed a
"head-fold" and a "tail-fold." A further fold, however, occurs at the
sides which bend in, so that the whole embryonic mass at this stage
comes to form an incomplete tube, the incomplete portion being the lower
aspect of that tube. This remains open. In due time this lower, or
ventral portion, becomes completely closed, except just at one point.
This point is where the communication exists between the inside of the
tube, which is the embryo, and the yolk sac. A part of the yolk sac is
thus included in the embryo itself, and this has an important bearing
upon future development, because in the course of time this part comes
to be the alimentary tract of the growing embryo. The canal which joins
the yolk sac to the internal gut of the embryo (the vitelline duct)
ultimately forms, together with part of the yolk sac, the umbilical
cord. This cord, which at the time of birth is artificially severed in
order to free the fully developed embryo, is at this stage connected to
the hinder part of the body of the embryo. As the latter grows, however,
it elongates still more behind, in what we should regard as the tail
region in animals which had a well-marked tail. As a matter of fact, at
a little later stage than this there is quite a conspicuous tail in the
human embryo, which, however, comes to be embedded in the tissues later
on, and so never forms any external appendage.
So that at this stage we have the embryo representing a mass of cells
which have gradually arranged themselves, and been arranged, in the form
of a tube more or less bent, and attached near its hinder end to the
tissues which are afterwards to represent the umbilical cord.
We have neglected to describe the organs and structures which are
developed after fertilisation as a further means of protecting the
developing embryo. We have done this of set purpose, because these
structures--known as the "trophoblast"--require a considerable amount of
technical knowledge to understand. Any detailed description of them,
therefore, would be out of place here. All that is necessary for us to
say is that they are intended to serve as a means of nutrition for the
developing embryo, and take no part in the actual formation of its cells
and organs. One portion of it, however, has another function which may
be mentioned. It secretes, it is supposed, a kind of ferment which has
the power of dissolving or digesting other cells, and this is of great
importance at one stage of development--namely, when the fertilised ovum
comes to reach the womb, or uterus, in which it is to pass the rest of
its developing stage. It is believed that some of the cells in the wall
of the uterus are dissolved and digested immediately round the ovum
itself, which thus comes to lie in a cavity in the uterine wall. This
process being carried still further allows the ovum to sink deeper and
deeper into the lining membrane of the uterus. Ultimately the point of
entrance, where the cells were digested, is closed up by the formation
of a clot of blood poured out at that spot, and which thus entirely
covers in the ovum. The latter now comes to lie absolutely embedded in
the wall of the uterus in a cavity which it has itself formed. It does
not, however, occupy the whole of the cavity, but is surrounded by blood
which is escaping from the minute blood-vessels of the wall in which the
cavity has been made. This blood is, of course, the maternal blood.
"Thus we have the ovum completely embedded, lying free in a tiny cavity
in the mucous membrane lining the uterus--a cavity full of blood, in
which the ovum lies bathed, and from which it presumably absorbs
nourishment by osmosis through its trophoblast." (R. W. Johnstone.)
The uterine wall, after this embedding of the ovum within it, undergoes
a remarkable growth at this position, concerning which a word must be
said. Under normal conditions this wall is smooth, or nearly so, but
probably there are upon it some slight irregularities or projections
which are sufficient to catch the ovum when it enters the uterine
cavity. Apparently it may be arrested in this way at any part of the
wall, and at that spot it becomes embedded in the manner we have
described above. The lining membrane of the uterus under ordinary
conditions measures about one-eighth of an inch in thickness, but, after
the ovum has become embedded in it, it begins to increase until it
reaches as much as half an inch. Underneath this lining membrane lies
the muscular part of the uterine wall. The ovum itself is embedded about
the middle depth of the lining membrane, but as it continues to grow,
and increases in size and dimensions it projects more and more into the
uterine cavity, that being the direction of least resistance. Before
very long the embryo, as it now is, has reached such a size in its
growth that it entirely fills the cavity of the uterus. This stage is
reached after the third month of gestation.
Another structure, concerning which just a word must be said, is that
known as the "placenta," or more commonly as the "after-birth." We need
only say that this is first developed by means of a number of little
outgrowths by means of which the early embryo is attached to the wall of
the cavity in which it lies. These outgrowths grow into the uterine
tissue around the ovum, and they allow of blood circulating between
them. They have, as a matter of fact, two distinct functions to
perform--first, that of fixing the ovum in position, and, secondly, they
allow of the maternal blood circulating in the spaces between them, and
it is from this blood that the embryo derives its nourishment. The
blood-vessels ultimately connect with those of the umbilicus, and thence
reach the embryo. This organ, the placenta, at the time the embryo is
fully developed at birth, is a round structure about nine inches across,
and not quite an inch thick in its middle, becoming thinner towards the
edges. The surface of it next to the infant is smooth and shiny, beneath
which it is rough, that next to the maternal structures being
dark-coloured, somewhat like flesh. When the child is born, the severing
of the umbilical cord allows the placenta to remain behind in the
uterine cavity, whence it is usually expelled shortly afterwards.
Should, however, this not be done, and the embryo and the placenta be
born together, the child is said to be "born with a caul," an event
which has given rise to many superstitions.
The foregoing description of the principal events in the development of
the embryo will be sufficient for our purpose here. Further details on
the subject would necessitate a considerable knowledge of physiology and
anatomy, and those readers who desire to study the details of the
subject further may do so in any of the various works referred to in the
bibliography appended to this book.