THE OVUM AND ITS DEVELOPMENT.
The ovum at maturity (and not impregnated) is described as being
composed of the vitelline membrane, which seems like albumen in
appearance, but is a thick, transparent membrane, without determinate
texture; second of the vitellus or yolk, a granular liquid contained in
the vitelline membrane, composed of a coherent transparent viscid mass;
third of the germinal vesicle, which is composed of a transparent
colorless membrane, enclosing a liquid also transparent; and lastly, of
the germinal spot, that is held in suspension in the liquid that the
germinal vesicle contains.
The ovum passes slowly through the Fallopian tubes, and during the
twelve days or more that it is passing to the uterus, there is some
development, some increase in size, and by the time it has reached the
uterus it has become impossible to find in it either vesicle or germinal
spot.
It is probable that in its passage it is nourished by the granulations
which accompany it, and by absorbing the liquid secreted by the oviduct.
As the impregnated ovum is developed, the chorion, the amnion and the
embryo may be observed.
The _chorion_, which corresponds to the membrane lining in the shell of
an egg, is found covering the ovum at the earliest period that it has
been seen in the uterus. It is smooth internally, but externally it is
covered with short round villi, which at a later period remain only
where the placenta is developed. The chorion is enveloped in a great
measure by the reflected decidua; there is at the outset considerable
space between the two, mostly filled at first by the villi of the
chorion, though there may be between the two an effusion of blood; these
villi soon disappear, and the membranes come in contact.
In that part of the chorion that is not covered by the decidua reflexa,
the villi are more and more developed, and they contribute a most
important part in the formation of the placenta.
At the same time that the placenta is formed, the villi on the other
portion of the chorion is obliterated, so that the principal part of the
chorion is a thin, colorless, transparent membrane, united outwardly to
the reflexed decidua by short, delicate filaments, and inwardly to the
amnion by an albuminous layer called the _tunica media_.
There is also between the two membranes the _vesicula alba_. This bears
a perfect analogy to the yolk of an egg; it is the vitellus surrounded
by the blastoderm. Its use is to contain nutriment for the fœtus before
the development of the placenta.
The amnion is the most internal membrane of the ovum; it is continuous
with the margins of the ventral opening in the fœtus, and closely
envelopes the embryo in the early period. Its internal surface exhales a
liquid into its own internal cavity, and in this the embryo swims
freely.
As this membrane is more and more filled, it presses back the exterior
liquid and thereby condenses it until the amnion comes in contact with
the chorion. And since it adheres to the abdominal parietes of the
fœtus, it furnishes as it extends a membranous sheath to the allantoid
and umbilical vesicle, and these vessels, and all parts thus enclosed
constitute the _umbilical_ cord.
The _placenta_ is an appendage of the chorion; it is a soft, spongy
mass, constituting the principal connection between the ovum and the
uterus, being destined to the hematosis and (as we suppose) to the
nourishment of the fœtus. The placenta, at the termination of utero
gestation, is a flattened body about an inch in thickness at the center;
its shape is circular or oval, and it is from six to eight and one-half
inches in diameter; its internal surface is covered by the chorion and
amnion, and exhibits plainly the umbilical arteries and veins which
converge to form the umbilical cord. Its fœtal portion is formed by the
hypertrophied villi of the chorion, with which its circumference is
continuous, and its maternal portion is continuous with the decidua, and
is in fact a thickened part of that membrane. As the villi of the
chorion are developed on one part of its surface, they ramify and form
filaments that engraft themselves upon the uterine mucous membrane and
adhere closely. At the same time there is an inverse development of the
uterine vessels, which form vast numbers of loops that descend between
the villi of the chorion, and extend through to the fœtal surface of the
placenta. An amorphous matter is soon thrown out which unites the two
parts together.
The placenta is, therefore, composed of two parts, distinct in their
physiological action, though they together present but one mass to our
view. One part is the fœtal portion formed from the chorion; the other
is the maternal portion formed from the uterine mucous membrane, of
which it is a greatly thickened part. After delivery the fœtal placenta
comes entirely away with the epithelial layer of the placental decidua,
and the placental distribution of the maternal vessels; a portion of the
maternal vessels remains attached to the uterus.
The placenta may be inserted upon any part of the uterine cavity,
although it is most usually near the fundus where the ovum must enter
the womb. If, as is sometimes the case, it is attached at the lower
part, over the orifice of the womb, it causes unavoidable hemorrhage in
the later months of pregnancy.
The _umbilical cord, funis, or naval string_, is the connecting link
between the child and mother. It commences when the external lamina of
the blastoderm with the alantois are so changed as to form a mere cord
upon which the two umbilical arteries ramify, and when all these have an
enveloping sheath from the amnios. It may be discerned in this state at
the end of the first month; at that time the fœtal intestines may be
seen to protrude beyond the umbilicus into the amnionic sheath, but the
cord is then cylindrical and very small. There are progressive changes,
the cord becomes simplified, the canal of the amnionic sheath is
obliterated gradually at its extremity, and as the effacement proceeds
towards the umbilicus the intestine is pressed back so that no hernia
remains.
There are two arteries in the cord; these arise from the abdominal aorta
in the fœtus; they go by a flexed and tortuous course to the placenta,
where they ramify and are distributed. There is only one vein which
returns the blood from the placenta; there the radicles coalesce to form
the branches; these unite to form the umbilical vein. This is not as
flexuous as the arteries, which, being longer, wind around the one
venous trunk. After the third month these may be plainly seen in the
sheath imbedded in what has been called _Wharton’s gellatine_.
Ordinarily the cord lies free and loose in the cavity of the amnion, but
occasionally owing to the movements of the child it may be coiled around
the child’s neck, be tied in knots, or it may escape below the head so
as to prolapse during labor.
The length of the cord varies; it is very rarely less than eight inches,
and it is sometimes six or eight feet long.
After the birth of the child, the pulsation in the cord ceases within
about fifteen minutes. After the cord is cut that portion that is
attached to the umbilicus dies and usually falls off about the fifth
day.
The blood of the fœtus is ærated or undergoes a change in the placenta
analagous to the change that our blood undergoes in the lungs.