HOW THE EMBRYO IS NOURISHED
Having noted how the embryo itself takes its origin, and then studied
something of the beginnings of some of its most important parts, we may
now very briefly refer to the subject of its own nourishment. This has
more than a mere academic interest, because obviously the proper growth
and development of all the various tissues and structures in the embryo
must depend ultimately upon the nourishment with which they are
supplied. Their own inherent characters cause them to divide and
subdivide so as to give rise to the millions of cells which are required
to make the body, but these cells, in their turn, are dependent upon
outside sources for the nourishment which enables them to keep on
growing, or to maintain their full growth when they have arrived at that
stage.
Nature has made many varied arrangements for this nutrition during
embryonic life in different classes of animals. In some a considerable
quantity of yolk is so arranged with reference to the embryo that the
latter can draw upon it for some time for its supplies. This is the
case, of course, in birds, and in some reptiles. We need here, however,
only consider the case of the human embryo.
Three sets of structures are concerned in human embryonic nourishment,
namely, the Allantois, the Villi of the Chorion, and the Placenta.
The Allantois is developed in the form of a hollow bud from the
posterior part of the primitive alimentary canal, and ultimately comes
to form the umbilical cord, and the embryonic part of the placenta. It
is this structure, the allantois, which allows at a very early date of
the embryo establishing a blood-connection with the maternal tissues,
and hence it plays a very important part in the transmission of
nourishment to the embryo. Not only does it do this, but it allows of
the removal of waste products.
The villi of the chorion are outgrowths by means of which the very early
embryo attaches itself to the walls of the cavity, which it has made for
itself in the wall of the uterus. As they grow larger, these villi push
their way into many of the small blood-vessels in the uterine wall, and
so come to lie actually in a mass of blood from which they abstract the
elements of nutrition. At first the villi themselves contain no
blood-vessels. Nourishment passes through them by a simple process of
osmosis. Later on, vessels grow into the villi themselves. The nutriment
supply is secretion, in the first place, of the uterine glands, which
these villi easily absorb. This process takes place during the first two
or three months of embryonic life. At the end of this time most of the
villi disappear, and the few that remain take part in forming the
f[oe]tal or embryonic portion of the placenta.
After the third month the embryo is nourished by the placenta itself,
which is at this stage developed. As we have seen, it arises partly
from the villi of the chorion, which is its embryonic portion. The other
part of it is maternal in origin, arising from the portion of the
uterine wall which is immediately over the embryo. The connection
between this structure, the placenta and the embryo, is constituted by
means of the umbilical cord. The function of the placenta is partly to
supply nutrition, partly to serve as an organ of respiration for the
embryo, whose lungs are, of course, not functional, and partly it acts
in the same way as the kidney does in after life, by excreting certain
products. From the placenta the embryo derives those food elements at
first provided by the secretion of the uterine glands. Afterwards these
elements are supplied by cells which lie between the f[oe]tal villi
and the blood of the mother. Its respiratory function consists in
allowing oxygen and carbonic acid gas to pass by osmosis between the
embryonic and the maternal blood. The process is exactly analogous to
that which takes place between the gills of a fish and the water in
which the fish lies. Of course, it will be easily understood that there
is as yet no great need for a large supply of oxygen, because the embryo
is merely growing, and not using its various organs.
It should be clearly understood that under ordinary conditions of
embryonic life there is no direct mixture of the blood of the mother and
that of the developing embryo. All the processes which contribute to its
growth and maintenance, including those of respiration and excretion,
take place through the intermediate structures above mentioned. This is
an extremely important point, because it means--and evidently that is
the object of the arrangement--that there may be much of an injurious
character in the blood of the mother which never reaches the embryonic
tissues at all. Doubtless the cells which form the organs of nutrition
for the embryo have a capacity for selecting the elements required for
purposes of nutrition. It is their business to look after this process.
How perfectly it is performed can at once be understood when we
recollect how very frequently the tissues of the mother herself are in
anything but perfect health, and yet the embryo is born healthy. Were it
not for this intermediary process, the embryo could hardly help being
poisoned or otherwise injured by all the varied unhealthy products and
substances which the ignorance of some mothers allows to be present in
their blood during this important period. Even with this means of
protection, the maternal blood may be so utterly deficient in nutritive
qualities, or so actively injurious from saturation with alcohol, or
from some equally toxic substance, that the fluids which reach the
embryonic cells may be very much impaired in quality. Nevertheless, it
is astonishing how much danger can be avoided in this way by Nature's
provision in the method of nourishing the embryo.
If the development and growth of the embryo in a human being runs a
perfectly normal and uninterrupted course, the following points could be
observed at various stages. At the end of the fourth week in growth, the
embryo is distinctly curved, so that the two ends--the head and the
tail--are close together, the whole being about half an inch in length.
Even at this very early stage, the canal which gives rise to the brain
and spinal cord is closed in. The vesicles of the eye and the ear have
both made their appearance, and the limbs are just beginning to show as
buds. The heart is quite obvious, and its division into its four
chambers is commencing. In another four weeks the embryo has reached the
size of one inch, and the head is beginning to take on a shape more
resembling that associated with a human being. The tail, on the other
hand, has now disappeared. The limbs have grown to the extent that both
hands and feet are starting growth, and in the region of the head both
the eyes and the ears, as well as the nose, can be distinguished. Even
at this stage, however, the sex of the embryo cannot be made out. A
month later, at the end of the twelfth week, a considerable development
has taken place. The embryo is now about three and a half inches long.
There is a general growth to be observed, and the bones are beginning to
ossify. In sixteen weeks, when the embryo measures about five inches in
length, the sex is easily distinguishable. The most characteristic thing
for the weeks succeeding this is the relatively large size of the head,
upon which hair appears at about the twenty-fourth week. In twenty-eight
weeks the embryo should weigh about 2-1/2 lb., that is to say at the
seventh month of embryonic life. Should the child be born at this time
as the result of any of the causes which give rise to premature birth,
there is a possibility that it may live, though as a rule it does not.
Four weeks later it should weigh 3-1/2 lb., and if born now may frequently
live, if carefully attended to. In another four weeks the embryo is
nearly eighteen inches long, and weighs about 5-1/2 lb., and the body has a
more rounded appearance, because by this time there has been a
considerable growth in fat. If born at this stage it ought to be quite
possible to save the life. Finally, at the end of forty weeks, the
normal full embryonic period of human life, the healthy child should
weigh about 7 lb., having smooth, pink skin, and being otherwise
perfectly developed.