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biology 0610printed 215 · PDF 215
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Implantation
● After fertilisation a zygote forms.
● It slowly continues to move down the oviduct.
● As it goes down it divides repeatedly and ultimately forms a ball of cells called
an embryo.
● The embryo contains nutrients from the yolk of the egg.
● It takes several hours for the embryo to reach the uterus. The uterus has a
thick and spongy lining and the embryo sinks into it. This is called
Implantation.
Placenta
● The embryo continues to divide. As the embryo grows a placenta also grows
which connects it to the walls of the uterus.
● The placenta is soft, dark red and has finger-like projections called villi.
● The placenta is the place where substances are exchanged between the
mother’s blood and the embryo’s blood.
● After 11 weeks, the embryo has grown most of its organs and is called a
fetus.
● The placenta is joined to the fetus by the umbilical cord.
● Inside the cord are two arteries and a vein. The arteries take blood from the
fetus into the placenta and the vein returns the blood to the fetus.
● The placenta contains capillaries filled with fetus blood. The lining of the
the uterus contains large spaces filled with the mother’s blood. The fetus’s and
mother’s blood does not mix. They are separated by the walls of the placenta.
● Oxygen and dissolved nutrients in the mother’s blood diffuse across the
placenta into the fetus’s blood and are then carried along the umbilical cord to
the fetus.
● Carbon dioxide and other excretory products diffuse in the other direction
and are carried away in the mother’s blood.