Fertilisation and the growing embryo

The meeting of sperm and egg

During mating, sperm are released into the vagina. They swim up through the cervix and the uterus into the oviducts. If an egg is there, one sperm may fuse with it. This joining is fertilisation, and it happens in the oviduct.

The fertilised egg is called the zygote. It is a single cell, with DNA from both the mother and the father.

The zygote divides

The zygote starts to divide at once. One cell becomes two, two become four, and so on. As it divides, it moves down the oviduct towards the uterus. The growing ball of cells is an embryo.

You should find that each division doubles the cells: after $3$ divisions there are $8$ cells, and after $10$ there are $1024$.

Settling in the uterus

Each month, the uterus prepares to receive a zygote. Its inner lining becomes thick and spongy, with a rich supply of blood. The embryo reaches the uterus and sticks to this lining. This is called implantation.

The embryo now needs food and oxygen, and it must get rid of its wastes. It does this through a special tissue, the placenta.

• The placenta is a disc embedded in the wall of the uterus.
• On the embryo's side it has finger-like projections, the villi, which give a very large surface.
• On the mother's side are spaces filled with her blood around the villi.
• Glucose and oxygen pass from the mother's blood to the embryo.
• The embryo's wastes pass the other way, into the mother's blood, to be removed.

The blood of the mother and the blood of the embryo do not mix. Substances pass across the thin walls between them.

About nine months

The embryo grows and develops its organs. It takes about nine months inside the mother. At the end, the muscles of the uterus contract rhythmically, and the baby is born.

When the egg is not fertilised

Most months the egg is not fertilised. The egg lives for only about a day. The thick, spongy lining is then not needed. It slowly breaks down and comes out through the vagina as blood and mucus. This is menstruation.

• It happens about once a month, so the whole cycle is called the menstrual cycle.
• Each menstruation lasts for about two to eight days.
• Then the lining builds up again, ready for the next egg.

Menstruation is a normal, healthy part of growing up. It shows that the body is working as it should.

Try it in Python

The program follows the zygote's first eight divisions.

You should see $2, 4, 8, \ldots$ cells, and $32$ cells after the fifth division.

Practise with new numbers

Common mistakes

• Placing fertilisation in the uterus. It happens in the oviduct; the embryo settles in the uterus later.
• Thinking the mother's blood flows into the embryo. The two bloods stay apart; substances pass across thin walls.
• Thinking the zygote is already a baby. It is a single cell that divides many times.
• Seeing menstruation as illness. It is the normal shedding of a lining that was not needed.

Remember these

• Fertilisation: a sperm fuses with the egg in the oviduct, forming a zygote.
• The zygote divides, moves to the uterus and settles in the thick lining (implantation).
• The placenta passes glucose and oxygen to the embryo and takes away its wastes.
• Development takes about nine months; birth comes with rhythmic contractions of the uterus.
• No fertilisation: the lining breaks down, and menstruation follows, about once a month.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Where does fertilisation take place?

2. What passes from the mother to the embryo through the placenta?

3. Why does the lining of the uterus become thick and spongy each month?

4. A zygote has divided $6$ times. How many cells does it have?

5. What happens to the lining of the uterus when the egg is not fertilised?