The early sequence of human development runs in a fixed order: sperm released, fertilisation in the oviduct, division by mitosis, implantation in the uterus lining, then placenta and fetal growth. Exam questions ask you to place these stages in order, name where each happens, or explain why one follows another.
This lesson belongs to human reproduction and development and leads into the structures of the placenta and reproductive system.
What happens at each stage?
1. Sperm enter the female system. Sperm are released into the vagina and swim through the cervix and uterus towards the oviduct. Only a small fraction reach the egg.
2. Fertilisation. In the oviduct the nucleus of one sperm fuses with the nucleus of the egg cell. Each gamete is haploid, with 23 chromosomes in humans. The fused cell, the zygote, is diploid, with 46 chromosomes.
3. Cell division. The zygote divides by mitosis repeatedly as it moves down the oviduct. It becomes a ball of cells called an embryo. Each cell keeps the same genetic information.
4. Implantation. The embryo reaches the uterus and sinks into its soft, thick lining. This is implantation. The lining has been prepared by hormones and is rich in blood vessels.
5. Placenta and growth. The placenta forms where the embryo attaches. The embryo, later called a fetus, is connected to it by the umbilical cord, and grows inside the amniotic sac.
A quick way to remember where
Link each stage to one of three places: oviduct (fertilisation and early division), uterus (implantation and growth), placenta (exchange). If a stage has the wrong place attached, the answer is wrong even when the order is right.
Worked example
Question: Four statements are given in the wrong order. Put them in the correct sequence.
- A. The embryo sinks into the lining of the uterus.
- B. The zygote divides by mitosis into a ball of cells.
- C. A sperm nucleus fuses with the egg nucleus.
- D. The placenta forms and exchange begins.
Step 1. Find the start. C is fertilisation, which creates the zygote, so it comes first.
Step 2. B follows, because the zygote must exist before it can divide.
Step 3. A follows B, because implantation needs an embryo that has reached the uterus.
Step 4. D comes last, because the placenta forms only after the embryo has attached.
Answer: C, B, A, D.
The mistake to watch for
Mistaken answer: “Fertilisation happens in the uterus, and then the embryo implants.”
This places fertilisation in the wrong organ. It also hides a gap: the zygote must travel from the oviduct to the uterus while dividing. The correction is to attach a place to every stage: fertilisation in the oviduct, implantation in the uterus lining.
A second slip is saying the zygote divides by meiosis. Meiosis makes the gametes. Growth of the embryo uses mitosis.
Check yourself
1. Name the cell formed at fertilisation and state how many chromosomes it contains in a human.
Show answer
The zygote, with 46 chromosomes (it is diploid, made from two haploid gametes with 23 each).
2. Explain why the embryo is genetically identical in all its cells.
Show answer
The zygote divides by mitosis, and mitosis produces daughter cells with the same genetic information as the parent cell.
3. State the difference between fertilisation and implantation.
Show answer
Fertilisation is the fusion of the sperm nucleus and egg nucleus, usually in the oviduct. Implantation is the later attachment of the embryo to the lining of the uterus.
Where this leads next
Next, label the structures involved in explaining the diagram, and practise everything together in the mixed practice set. The inheritance model board shows what the zygote’s genetic information means for a simple cross.
Some students know each stage but lose marks by missing the place or the reason. Our teachers work on that gap in online one-to-one Biology tuition.