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Lesson: Chapter 14 — Continuity of Life

14.3. Fertilisation, pregnancy and birth 1 of 5

Fertilisation and implantation

In intercourse, semen is released into the vagina.

  • The sperm cells in it, present in very large numbers, swim in the fluid and travel through the uterus towards the upper part of the fallopian tube.
  • Then one sperm joins with the ovum coming along the fallopian tube towards the uterus.
  • Here the nuclear material of the ovum and of the sperm cell join together. This is called fertilisation.

Figure to be added

Figure 14.28 — a sperm and an ovum at fertilisation.

From zygote to implantation

The fertilised ovum is called the zygote. It travels along the fallopian tube towards the uterus, dividing and increasing its number of cells. It is then called the morula.

The morula sinks into the tissue of the uterine wall, eroding it, and is deposited there. This is called implantation.

Figure to be added

Figure 14.29 — the stages from fertilisation to implantation: day 1 (the zygote), day 2 (2 cells), day 3 (4 cells), day 4 (16 cells), day 5 (the morula) and day 7 (the blastula).

Where each event happens

Fertilisation happens in the fallopian tube, not the uterus. The first week is spent travelling — the embryo is already 16 cells or more by the time it reaches the uterus, and implantation is the end of that journey rather than the start of it.