THE DEVELOPMENT OF THE OVUM
The Germinal Cells--Fertilization--The First Steps in Development--
The Reaction of the Uterus--The Amniotic Fluid--The Placenta--The
Umbilical Cord.
Pregnancy, besides changing the external form of the body, causes
sensations--as for example those due to fetal movements--which are so
distinctive that they cannot escape notice. These obvious evidences
of approaching motherhood naturally lead thoughtful women to wonder
about the hidden mechanism of development, a mechanism which, of
itself, causes no sensation whatever. It is for this reason, perhaps,
that a prospective mother's imagination is so apt to be unusually
active, often picturing absurd conditions as responsible for one
symptom or another. Those who give free play to the imagination in
regard to the formation and progress of the embryo are pretty certain
to arrive at erroneous if not grotesque conclusions; for example,
they may attribute a protracted pregnancy to the child's having grown
fast to the mother, a situation that cannot arise.
Of course it is not essential that a prospective mother should
understand what is happening within the womb. And upon those who
prefer to be ignorant of the mechanism of development I would not
urge another point of view, for not ignorance but the unchallenged
acceptance of "half-truths" and of totally incorrect explanations is
the chief source of harm. On the other hand, my own experience has
taught me that women who wish to know about development should be
told the truth. In accord with this is the fact that I never have
more satisfactory patients than those who have previously been
trained nurses and who, in preparing for that profession, received
instruction concerning the reproductive function of human beings.
A description of development, in order to be perfectly clear, must
begin with a word about the fundamental structure of the adult body.
Everyone knows that the various parts of the body perform different
functions; but not everyone, perhaps, realizes that, in spite of
their different functions, all the organs of the body are composed of
similar structural units, known as cells. Of course, cells are
definitely arranged according to the use for which the tissue that
they chance to compose may be designed; they have, moreover,
distinctive individual peculiarities which can be easily recognized
under the microscope; but the essential features of the cells remain
the same, wherever they may be located. That is to say, each cell is
a minute portion of living matter, or protoplasm, separated from its
neighbors by a partition, the cell-membrane; each has its own seat of
government, the nucleus, located near its center; and each, to all
intents and purposes, leads an individual existence.
THE GERMINAL CELLS.--Many of the cells in the human body are able to
produce others of their kind. This they do virtually by growing and
splitting in half; cell-division, as this splitting is called, really
represents reproduction reduced to the simplest terms. Most cells can
do no more than produce units like themselves. The bodies of women
contain, however, a type of cell which possesses a far more wonderful
power. Provided the requisite conditions for such development are
met, these cells are capable of developing into human beings. Each of
these remarkable units is called an Ovum, or egg-cell, and represents
one variety of the germinal cells. But the other variety, represented
by the Spermatozoon and developed only in the male sex, is also
required for the production of a human being.
Every ovum originates in the ovaries. These are organs peculiar to
women, having the size and shape of large almonds, and placed in the
lower part of the abdominal cavity. Though the ovaries are two in
number, one alone is sufficient for every requirement of health. It
has been estimated that the ovaries together contain at the time of
birth about 40,000 ova, distributed equally between them. Since less
than 500 ova are required to insure regularity in the menstrual
function, it is clear that, if the surgeon finds it necessary to
remove one of the ovaries, the other will provide abundantly for
menstruation and for the bearing of children. Although every ovum
that will be produced as long as a woman lives has already sprung
into existence by the time she is born, not a single one ripens for
from twelve to fifteen years. The ripening process begins about the
time of puberty, and, unless suspended through the occurrence of
pregnancy, continues until the menopause. During this period, which
is also characterized by the periodical appearance of menstruation,
one ovum ripens each month; sometimes, though rarely, several ripen
at once, and this tendency is partly responsible for twins.
The human ovum is a tiny structure, measuring about 1/125 of an inch
in diameter. With the naked eye it can barely be seen; magnified by
the microscope it appears as a little round bag made of a transparent
membrane. Briefly described, the ovum is a single cell. That is, it
belongs to the simplest class of anatomical structures, and is one of
the millions upon millions of units that make up the body. It
contains a nucleus surrounded by nutritive material, the yolk. Yet
the quantity of yolk is exceedingly small. In this particular the
human ovum differs notably from the egg of birds, as it does also in
that it lacks a shell. Obviously, a shell would not only be useless
to an embryo developing within the body of its parent, but would shut
off the nourishment, which, since the ovum contains so little, must
necessarily be provided by the mother.
When the ovum has ripened, it becomes detached from the ovary, and
enters a fleshy tube about the size of a lead pencil, known as the
oviduct. There are two of these tubes, one running from the
neighborhood of each ovary; both enter the uterus, but on opposite
sides. The ovum travels down the tube which corresponds to the ovary
where it originated. The journey is fraught with momentous
consequences, for it is during this passage through the oviduct that
the fate of the ovum is determined. If it is to develop into a living
creature, a great many conditions must sooner or later be fulfilled;
but there is one which must be promptly satisfied. Shortly after
leaving the ovary the ovum must receive the stimulus to live and
grow; otherwise it will quickly wither and die. This vital stimulus
can be imparted only by the spermatozoon.
The male germinal cell is like the female cell in the possession of a
nucleus; in other respects it is very different. Longer but much
narrower than the ovum, the tiny arrow-shaped spermatozoon is
particularly distinguished by its active motility, for it has a tail
that propels it. The human male cell must travel some distance to
reach the point where it can meet a ripe and vigorous ovum; and since
the journey is not without danger to its life, Nature has provided
that exceedingly large numbers of the male cells shall be deposited
in the vagina at the time of the marital relation. In this way, it is
made sure that some of them will travel up through the uterus and
oviducts, arriving in the neighborhood of the ovaries.
FERTILIZATION.--Convincing observations upon the lower forms of life,
especially upon fishes, have shown that when the germinal cells come
near to each other, the ovum attracts the spermatozoon. The power of
attraction which the ovum exerts may be likened, most simply, to the
influence of a magnet upon iron-filings. While there has been no
opportunity to observe such attraction between the parent cells of
human beings, its existence is not open to doubt. And it is
practically certain that these cells meet in the oviduct, even in
that portion of it which receives the ovum just as it leaves the
ovary. Thither a number of the male cells have traveled by their own
activity; several come in contact with the ovum and one, but only
one, actually enters it. Almost at the moment when they touch, the
two cells unite so intimately that all trace of the spermatozoon is
lost. Fertilization of the ovum, as this event is scientifically
termed, has as its main purpose the uniting of the nucleus of a male
germinal cell with the nucleus of the female germinal cell. This
detail has been carefully studied; we know that the nuclei quickly
blend into one, and that the particles of living matter contributed
by the male animate the female cell with a new and wonderful
activity.
In our every-day way of speaking, fertilization means conception; it
is the instant in which a living being begins its existence. There is
no longer the slightest excuse for confusion regarding the period at
which the life of the unborn child begins. Before the significance of
fertilization was understood, it was perhaps not unreasonable to
believe that life began with quickening or about the time the fetal
heart-sounds could be heard. But now we must acknowledge that both
these ideas were incorrect. The animation of the ovum at the moment
of conception marks the beginning of growth and development which
constitutes its right to be considered as a human being.
Individuality, hereditary traits, sex--all these, we may be sure--are
unalterably determined from the moment of conception. The germinal
cell forms the total contribution of the male parent to pregnancy;
therefore no other opportunity for him to influence his progeny
presents itself, and the substance which enters the ovum at the time
of fertilization must be the basis of inheritance from the father. It
is equally true, as we shall see in the next chapter, that the
nucleus of the ovum and the nucleus alone transmits maternal
qualities. The material which conveys inheritable characters can be
seen and has been identified in both germinal cells; from each of
them the fertilized ovum derives equal amounts. As the parental
nuclei unite, the material which they contain intermingles and
establishes a new being; to attain full development, it requires
nothing further from the father, and nothing save nourishment from
the mother.
THE FIRST STEPS IN DEVELOPMENT.--Although the identity of the
spermatozoon is lost at the moment of fertilization, its influence
just then begins to be asserted. In the fertilized ovum the dawn of
development is shown at first by unusual activity within and later by
alterations upon the surface. Before very long the circumference of
the cell becomes indented as if a knife had been drawn around it, and
shortly two cells appear in place of one. These two cells in turn
divide, yielding four cells which grow and divide into eight. In this
manner division follows division until a multitude of cells have
sprung into existence, all of which cling together in the shape of a
ball. Development always proceeds in the same orderly way; evidently
it is governed by fixed laws which decree that the mass shall remain
for a while in the form of a ball, though the ball, at first solid,
soon becomes hollow.
While these changes are taking place the growing ovum is carried down
the oviduct a distance of four to six inches and finally comes to
rest in the uterus, where it is to dwell during the months necessary
to its complete development. The time consumed by this journey cannot
be measured accurately; it may be as short as a few hours or as long
as several days, but in all probability it is never longer than a
week. Although the element of time is uncertain the method of
transmission is well understood. Of its own accord the ovum can move
after fertilization no better than before; it is never capable of
moving itself. The active agent of transportation is the oviduct,
which has been fitted for this purpose with millions of short, hair-
like structures that project into its interior. These are closely set
upon the inner surface of the oviduct; their outer ends are free and
continually sway to and fro like a wheat field on a windy day; and by
their motion they create a current in the direction in which the ovum
should move, namely, toward the uterus. While passing through the
oviduct, the ovum has no attachment whatever to the mother, yet
development is going on all the time. It is thus made perfectly clear
that development is not directed by the parent. This independence of
the parent, though it continues to be one of the characteristic
features of the development of the ovum, shortly becomes less
evident, for communication is set up between the mother and the ovum
as soon as it reaches the uterus. Unless we were warned, we might
easily misinterpret the significance of this attachment to the
parent. It does not permit the mother, for instance, to influence the
mind or character which the child will have. The purpose of the
attachment is twofold, namely, to anchor the ovum, and to arrange
channels by which, on the one hand, nutriment may reach the embryo,
and, on the other, its waste products may return to the mother. The
mother may influence the nutrition of the fetus; but she cannot
determine the kind of brain or liver her child will have; neither for
that matter can she alter the development of any portion of the
embryo.
After its entrance into the cavity of the uterus prepared to receive
and protect it, the mass of cells sinks into the soft, velvety lining
of the organ. Here it is entirely surrounded by tissue which belongs
to the mother. But just before implantation takes place the
architecture of the ovum is modified in such a way as to indicate the
trend of its subsequent development. We left it, a hollow ball
passing down the oviduct; had we examined the sphere more closely we
should have found its wall composed of a single layer of cells. At
one spot, however, the wall soon thickens. The thickening is due to a
specialized group of cells which gradually grows toward the hollow
center of the ball. A little later, if we study the structure as a
whole, we find it a small, distended sac, from the inner surface of
which hangs a tiny clump of tissue. The clump of cells within and the
inclosing sac as well are both requisite to the ultimate object of
pregnancy; yet they fulfill very different purposes. The clump within
will mold itself into the embryo; the inclosing sac will make
possible the continued existence and growth of the embryo by securing
and conveying to it nourishment according to its needs. These two
structures, which from now on constitute the ovum, can best be
considered separately and in the order of their development. We shall
therefore first study the sac and in the next chapter the embryo.
For a time after this sac, or ball, as you may choose to think of it,
becomes implanted in the uterus, every part of its wall shares in the
responsibility of procuring nourishment for the embryo. On this
account the wall, or capsule, is for several weeks the most
conspicuous part of the ovum. Its position is naturally advantageous,
for, since it forms the outermost region of the structure and comes
into immediate contact with the tissues of the mother, it has the
first opportunity to seize and appropriate nutriment. Consequently,
while there is still relatively little development in the embryo, the
capsule of the ovum gives evidence of rapid extension; the wall
becomes thicker, and the circumference of the sac increases. The
significant thing about this growth, however, is the fact that it
does not progress evenly. At some points cell-division is more active
than at others, with the result that the surface of the ovum speedily
loses its smooth, regular outline. Projections from the capsule
appear; they increase in number and in length; and by the end of four
weeks the ovum, as yet less than an inch in diameter, resembles a
miniature chestnut-burr. To make the comparison more accurate, we
must imagine such a burr covered with limp threads instead of rigid
spines.
These projections, the so-called Villi, push their way into the
mucous membrane of the uterus and serve a two-fold purpose. One of
their functions is to fix the ovum in its new abode; and, though the
attachment is not at first very secure, it becomes stronger in the
course of time and is capable of withstanding whatever tendency the
activity of daily life may have to loosen it. The other, and equally
important, task of the villi, the majority of which dip into the
mother's blood, is to transmit substances to and from the embryo.
We have traced thus far the earliest steps in the development of the
ovum. One portion, we observed, was promptly set apart for the
construction of the future child; this favored portion became
inclosed by all the rest of the ovum, which has a more or less
spherical form and is technically called the fetal sac. The first
duty of the sac is to take root in the womb, and the second, no less
vital, is to draw nourishment from the mother. But neither of these
functions can be performed without the participation of the uterine
mucous membrane, the soil, as it were, in which the ovum is planted.
We must now learn how the maternal tissues assume the responsibility
placed upon them.
THE REACTION OF THE UTERUS.--The womb, which is small before
marriage, is converted by pregnancy into the largest organ of the
body. The virginal uterus, shaped somewhat like a pear, and placed
with apex downward, is carefully protected within the bony basin
between the hips, which is commonly called the Pelvis. The upper and
larger part of the organ, known as the body, lies at the bottom of
the abdominal cavity; the lower part, the neck, projects into the
vagina. The cavity inside the womb communicates above with the two
oviducts and terminates below in a canal which runs through the neck
and opens into the vagina by an orifice known as the mouth of the
womb.
Pregnancy modifies every portion of the womb in one way or another;
but the most profound alterations occur in the body, in the cavity of
which the ovum has come to rest. During the forty weeks of gestation
the organ grows in weight from two ounces to as many pounds; from
three inches in length it increases to fifteen inches; and its
capacity is multiplied 500 times.
The mucous membrane which lines the cavity of the uterus responds to
the stimulus of pregnancy in a characteristic manner and with a
single purpose, namely, to promote the development of the ovum. In
connection with menstruation we noted that this membrane periodically
prepares for the reception of an ovum. And if the expected ovum has
been fertilized, its arrival is followed by arrangements for its
protection and nutrition which are far more elaborate than the
preparations for its reception. Within a few weeks the mucous
membrane becomes half an inch thick, that is, about ten times thicker
than it was; and all the elements entering into its composition,
become unusually active. The blood-vessels are congested; the glands
pour out a more elaborate secretion; and certain cells lay up a
bountiful store of material to be drawn upon in the formation of the
embryo and the building up of the structures that promote its
development.
The ovum is as likely to find a resting place at one spot as another
upon the surface of the uterine mucous membrane. The whole of that
surface has been made ready to receive it; yet the area actually
required to imbed the tiny object is extremely small. As the ovum
escapes from the oviduct and enters the womb, it is smaller, in all
probability, than the head of a pin. For at least a week after its
coming, diligent search is necessary to find the site of
implantation. Insignificant as it is at first, however, the region of
implantation later becomes very prominent, for it undergoes a
transformation that the rest of the mucous membrane does not share.
That is to say, it becomes the point of attachment of the Placenta,
an organ that has the very important function of drawing upon the
resources of the mother's blood. As the ovum sinks into this
especially prepared bed, the villi are formed. They break open the
adjacent capillaries of the mother, thus diverting her blood from its
accustomed course. The blood collects in microscopic lakes in contact
with the capsule of the ovum, and from them flows back into the
mother's veins. Through the veins it returns to her heart, by which
it is distributed through the arteries to the various regions of the
body. The tiny lakes, in which the villi hang, are thus made a part
of the mother's circulation and as such are regularly replenished
with purified blood. By this means the ovum receives a rich supply of
nutriment, and as a natural consequence its growth is rapid.
Before very long the diameter of the ovum is greater than the depth
of the mucous membrane which surrounds it. Consequently that part of
the membrane which covers it is pushed into the uterine cavity, as
the ground is raised by a sprouting seed. Growth continues, the
bulging increases, and extensive alterations are wrought both in the
womb and in the capsule of the ovum. One of these alterations will be
more easily understood if we still think of the ovum as a seed, for
it grows away from its roots just as plants do. Most of the capsule,
therefore, is removed step by step farther from its source of
nourishment, for the maternal blood-vessels do not follow the
expanding sac but retain their original position at its base. Partly
on account of the lack of nutriment thus occasioned and partly on
account of the distention caused by the contents of the sac, atrophy
occurs in the distant portions of the sac's wall. As a final result
of these two factors, the maternal tissue which covers the ovum
becomes thinned and stretched; it is pushed entirely across the
uterine cavity; and by about the twentieth week meets the opposite
side of the cavity, to which it becomes adherent. Subsequently, the
sac which incloses the embryo becomes everywhere fastened to the
inner surface of the uterus and completely fills the uterine cavity.
THE AMNIOTIC FLUID.--The great enlargement of the uterus which is so
marked a characteristic of the latter part of pregnancy is due in a
measure to the luxuriant blood-supply, for better nutrition always
causes growth. In a far larger measure, however, it is due to
distention for which the product of conception is responsible. Beside
the fetus the inclosing sac also contains a considerable quantity of
fluid. This fluid, called "The Waters" by those who have no special
knowledge of anatomy, is technically designated as the Amniotic
Fluid.
In the earlier months of pregnancy the amniotic fluid is not
abundant; later it increases rapidly, so that by the end of the
period it measures about a quart, and frequently even more. The
slightly yellow amniotic fluid is itself clear, but small particles
of dead skin and other material cast off from the surface of the
child's body are floating in it, and may cause turbidity. The absence
of odor supports the view that this fluid is not the child's urine.
The evidence thus far adduced, though not absolutely conclusive,
gives good reason to believe that "the waters" are secreted by the
inner side of the sac which incloses the fetus. Very early in
pregnancy this sac becomes a double-walled structure; and, though its
layers are intimately blended, and together measure not more than
1/16 of an inch in thickness, with a little care they can be
separated. The outer layer, which comes in contact with the inner
surface of the uterus and has to do with the matter of nutrition, is
called the Chorionic Membrane; the inner, the so-called Amniotic
Membrane, is much the stronger and is devoted to the protection of
the embryo, which it completely surrounds with fluid, at the same
time retaining the fluid within set bounds.
The amniotic fluid performs many important duties. Perhaps the first,
in point of time, is to provide sufficient room for the embryo to
grow in. Later, as the fluid increases, it permits the fetus to move
freely, and yet renders the movements less noticeable to the mother.
Again, the amniotic fluid prevents injuries that might otherwise
befall the child in case the mother wears her clothing too tight.
Harmful as the practice of tight-lacing during pregnancy is, it does
not, thanks to the presence of the amniotic fluid, result in the
disfigurement of the child. For the same reason a blow struck upon
the abdomen, as in a fall forward, is not so serious as might be
thought, since the fluid, not the child, receives the force of the
impact. Some physicians believe that the fetus swallows the amniotic
fluid and thus secures nourishment. The fluid also serves to keep the
fetus warm; or, to be more exact, protects it from sudden changes in
the temperature of the mother's environment. Normally the temperature
of the fetus is thus kept nearly one degree higher than the
temperature of the parent.
Ultimately, the amniotic fluid assists in dilating the mouth of the
womb, which remains closed until the beginning of the process that
terminates with birth. The uterine contractions at the onset of labor
compress the fluid; in turn the fluid attempts to escape but is held
in check by the amniotic membrane, which it drives into the canal
leading from the uterine cavity to the vagina. Acting like a wedge,
the fluid gradually pushes the mouth of the womb wider and wider
open, until it is large enough for the child to pass. The sac usually
ruptures when that point is reached, the fluid escapes, and in due
time the child is born. This is followed within half an hour by the
extrusion of a mass of tissue--in reality the collapsed fetal sac--
which in every language, so far as I know, is named the After-Birth.
An examination of this tissue at the time of delivery repays the
physician, for it is important to ascertain that none of it has been
left in the uterus. Our interest at present, however, is to learn how
the after-birth has assisted toward the growth of the child.
THE PLACENTA.--The after-birth has puzzled scientists as well as the
laity, and not until comparatively recent times have its origin,
structure, and use been satisfactorily explained. Its meaning
profoundly interested primitive men and stimulated their imagination
scarcely less than the mystery of conception. Some uncivilized tribes
believed that the after-birth was animated like the child;
consequently they spoke of it as "the other half," and often saved it
to give to the child in case of sickness. But generally the after-
birth was buried with religious ceremony, and was occasionally
unearthed later to discover whether the woman would have other
children; the prophecy was made according to the manner of
disintegration or some other equally absurd circumstance.
The after-birth consists of a round, fleshy cake, the placenta, to
which two very essential structures are attached. One of these,
running from one surface of the cake, is a rope-like appendage, the
umbilical cord, which links the placenta with the fetus. The other,
attached to the circular edge of the cake, is a thin veil of tissue,
in some part of which a rent will be found. Now, if we lift the
margin of the rent, we shall see that the veil and the cake together
form a sac which we are holding by the opening. This aperture through
which the fetus passed, and it was really made for that purpose, was
formerly placed over the mouth of the womb; the sac itself, distended
by the fetus and the amniotic fluid, was fastened everywhere to the
inner surface of the womb.
It is plain that we have now in our hands the fetal sac, the
development of which we have already traced from the beginning. The
wall of the sac, it will be recalled, was originally of the same
formation throughout; but when the ovum became imbedded in the womb,
that part of its capsule which remained in permanent contact with the
mother's blood underwent special development, whereas the rest of the
capsule gradually pushed away from its primary position and, becoming
stunted in its growth, even lost to some degree the development it
had attained. This latter portion, the veil that passes from the edge
of the placenta, is formed of the two membranes we have mentioned,
namely, the chorion and the amnion.
The placenta is, for the most part, a highly developed portion of the
chorionic membrane, which became specialized simply because it
happened to receive the best supply of blood. At the time of birth
the placenta measures nearly an inch in thickness, is as large around
as a breakfast-plate, and generally weighs a pound and a quarter,
that is, approximately one-sixth of the weight of the child. This
relation between the weight of the placenta and of the child is
regularly maintained; therefore, the larger the child the larger the
placenta associated with it.
The placenta has two surfaces, easily distinguished from each other.
The raw maternal surface was formerly attached to the inside of the
uterus; the fetal surface, covered by the amniotic membrane, was in
contact with the amniotic fluid. Across the fetal surface run a
number of blood-vessels containing the child's blood, converging
toward a central point at which the umbilical cord is inserted. The
point at which the cord is attached affords the simplest means of
distinguishing the two surfaces of the placenta.
Our knowledge as to how the exchange of food and excretory products
between mother and child is carried on by the placenta has been
gained chiefly through the microscope. The oldest medical writings,
as we might suppose, express very fanciful ideas regarding the nature
of embryonic development and the means by which it is made possible;
no rational view of these matters could exist until the circulation
of the blood was described by William Harvey in 1628. After this
epoch-making revelation, it was accepted as true that the mother's
blood entered the unborn child and returned to her own system. But
that view eventually became untenable, for it was proved conclusively
that there is no communicating channel between the two. For years
after that, it was believed that before birth the womb manufactured
milk to sustain the child, just as the breasts do afterwards; but
this theory also was disproved; and, as I have said, only by the use
of the microscope have we learned the truth about fetal nutrition.
When thin slices of the placenta are magnified they are found to
contain countless numbers of tiny, finger-like processes; these are
the villi, and they constitute the major portion of the organ. The
villi seen in a mature placenta are the same as those which projected
from the capsule of the young ovum, but not these alone, for many
branches have sprouted from the original projections. The primary
trunks with all their branches hang from the capsule of the ovum and
extract nutriment from the mother's blood which surrounds them, just
as the roots of a tree extract it from the soil.
The interchange of material between mother and child as carried on in
the placenta can, perhaps, be made clearer if we compare one of the
trunks and its branching villi to a human forearm, hand, and fingers.
The hand, we will imagine, is held in a basin of water, in which, by
turning on a spigot and leaving the outflow unstopped, we have
arranged that the water changes constantly. In terms of this
illustration, the water corresponds to the mother's blood, rich in
oxygen, mineral matter, and all other kinds of essential nutriment;
and the fingers are the villi. The blood-vessels in the fingers, to
go a step farther, represent the blood-vessels which exist within the
villi, connecting with the umbilical cord, and passing by that route
to the body of the child. The blood which thus circulates through the
villi, it is important to emphasize, is the child's blood; it cannot
escape through the coating of the villi, just as our blood cannot
escape through the skin of the fingers. Similarly, the mother's blood
cannot enter the child; the two circulations are absolutely separate
and distinct.
It must be noticed, moreover, that the maternal blood not only brings
to the surface of the villi everything the child needs, but it also
takes away the waste products of fetal life. Let us select one of the
foodstuffs necessary for the unborn child, and follow its course so
far as it relates to fetal nutrition. The mother's blood brings
sugar, for example, from her intestinal tract to the surface of the
villi; through the coating of the villi the sugar passes into the
fetal blood, is carried to the fetal heart, and distributed to the
various fetal organs. They burn it, deriving heat and energy, and in
return give off waste products, namely, carbonic acid gas and water,
which are taken up by the fetal blood, borne back to the placenta,
and pass again through the coating of the villi into the mother's
circulation. These waste products are then transported to the
mother's lungs and to her kidneys, and are finally thrown off from
her body. Before the child is born, therefore, the placenta, which is
an aggregation of villi, acts as its stomach, intestines, lungs, and
kidneys.
In every pregnancy the placenta serves in this way as an organ of
nutrition, arranging for the passage of food from the mother's blood
to the fetal circulation. Occasionally, it is interesting to observe,
the placenta performs a very different function, namely, the
protection of the unborn child from diseases that may attack the
mother. It is able to afford such protection, because the coating of
the villi is not permeable to all sorts of substances. In order to
pass through their walls, material must be in solution; solid bodies,
therefore, are denied admission to the fetal circulation. The most
significant result of this restriction is, perhaps, that so long as
the coating of the villi remains intact and healthful, bacteria
cannot gain access to the unborn child. Since in health there are no
bacteria in the mother's blood, this fact has no bearing upon the
average pregnancy; but in those exceptional cases in which typhoid
fever or some other infectious disease appears during pregnancy, it
is gratifying to know that Nature has provided an unusual defense
against infection of the unborn child.
That we do not know all about the interchange of substances between
mother and child must be admitted; but the essential facts, and they
alone are of interest here, have been established beyond contention.
There is no doubt whatever that the mother's blood surrounds the
placental villi but never enters the child. The fetal blood, on the
other hand, is first in the child's body, then in the villi, and then
returns to the child again. It never enters the blood-vessels of the
mother but passes to and from the placenta as long as pregnancy
lasts.
THE UMBILICAL CORD.--This rope-like structure, familiarly known as
the navel-string, which connects the placenta and the fetus, is
approximately twenty inches long; its length, therefore, is
sufficient to permit the newly born child to lie between the mother's
knees while the placenta remains attached to the womb. The cord is
about the thickness of the thumb and contains three blood-vessels,
all filled with fetal blood; in two of them the current is directed
toward the placenta, the third carries the blood back to the fetus
after it has circulated through the placental villi. In the cord the
vessels lie near together and are encased in a jelly-like substance
that protects them from injury.
So far as is known; the umbilical cord performs no service other than
to link the blood-vessels in the placenta with those in the fetus.
Simple as this may seem, it is of paramount importance in maintaining
the life of the fetus, for compression of the vessels in the cord
would shut off its nutriment. Against such accident, however, perfect
provisions have been made; both the amniotic fluid and the jelly-like
substance which surrounds the vessels are safeguards which
effectually protect the circulation from pressure that might
interrupt it.
Frequently, prospective mothers are told they must not "reach up" for
fear the cord will become entangled. Such a precaution is quite
unnecessary. No matter what the mother does, or does not, the cord
will be found around the child's neck at the time of birth in one of
every three cases. It is not difficult to understand how this
happens. The cord is longer than the uterine cavity and must fall in
coils toward the bottom of it. Now, since the fetus is free to move
it enters and withdraws from these loops, many times, in the course
of pregnancy. Finally, when it takes up a position head downward, as
it nearly always does, the head is the part of the fetus which passes
through the coil, should one happen to lie in its path. After the
head is delivered the physician always feels about the neck to
discover whether a loop of cord is there. If it is, he can release it
easily. This condition, since it occurs so frequently and since it so
rarely produces harmful consequences, should not be considered
unnatural.
After the child is born, the physician cuts the cord, and in due time
the after-birth is expelled through the same passage as was the
child. The expulsion of the after-birth frees the mother of all the
tissue derived from the growth of the ovum, for the intricate
mechanism that served to nourish and protect the embryo was almost
entirely developed from the ovum itself. It is a remarkable provision
of Nature that very little of the mother's tissue is cast off at the
end of pregnancy; and even this small portion is promptly replaced.
By about the sixth week after delivery, the wound which was made by
the separation of the fetal sac has completely healed. Meanwhile the
mucous membrane that underwent elaborate preparations to receive the
ovum, the cavity that was adjusted to its growth, and the muscle
fibers that were strengthened to insure its safe entry into the world
have all regained their original state. Except for the activity of
the breasts, the mother is left in the same physical condition as
before she became pregnant.