Structures and Processes of Life: Humans Fertilisation
Fertilisation is the fusion of the male sperm and the female egg to form a zygote. This takes place in the female’s fallopian tube.
Pregnancy
In the fallopian tube, the zygote immediately starts to divide to produce a ball of identical cells called an embryo.
The embryo reaches the uterus approximately five days after fertilisation. It is embedded into the capillary-lined wall of the uterus. This process is called implantation.
Sperm Egg z
Fig. 8.5.3 Fertilisation
8.5
Embryo
Fertilisation
Zygote
Cell division
Ball of identical cells
The female is considered to be pregnant when fertilisation is followed by the successful implantation of the embryo in the lined wall of the uterus.
A typical human pregnancy lasts for 40 weeks (about nine months). Over this period, the embryo develops into a foetus, ready to be born.
The placenta
Immediately after implantation, the placenta starts to develop. The placenta is a temporary organ that allows materials to pass between the mother and the foetus. The umbilical cord links the placenta to the foetus.
The embryo receives oxygen and food from the mother’s blood. Carbon dioxide and other wastes are also removed from the embryo via the mother’s blood.
The placenta is not a perfect barrier. As a result, the mother must avoid toxic chemicals, such as those found in cigarette smoke and alcohol, for the duration of pregnancy. These substances may harm a developing foetus.
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The implanted embryo passes the human chorionic gonadotropin (hCG) hormone into the mother’s blood. Pregnancy test kits can detect this hormone in the pregnant female’s urine. Test kits often indicate the positive presence of hCG by a colour change.
F z Fig. 8.5.4
The zygote divides to form the embryo
The embryonic period Over the next eight weeks, all organ systems continue to develop.
The embryo is surrounded by amniotic fluid within an amniotic sac. This sac provides a protective and temperature-stable environment.
