Internal Fertilisation and External Fertilisation: Differences Explained

What Is Fertilisation?

Fertilisation is the biological process in which a male gamete (sperm) fuses with a female gamete (egg or ovum) to form a zygote, the first single cell of a new individual. This fusion marks the start of development and ultimately leads to the formation of a new organism.

Depending on where this fusion happens, fertilisation may be classed as internal or external. The process of fertilisation varies from organism to organism.

What Is Internal Fertilisation in Humans?

In humans, fertilisation is internal, meaning it occurs inside the female body. During sexual intercourse, sperm are introduced into the female reproductive tract, travel through the cervix and uterus, and meet the egg in the fallopian tube. Fertilisation occurs in the ampulla, the wider section of the fallopian tube, where the sperm penetrates the egg’s protective layers to form a zygote.

After fertilisation, the zygote begins dividing, develops into an embryo and implants in the uterus. The rest of prenatal development takes place inside the uterus until birth. This process is known as viviparity in mammals.

What Occurs During Internal Fertilisation?

The internal fertilisation process in the human species involves several controlled steps:

• Insemination: Sperm enters the female reproductive system.
• Sperm transport: Sperm move toward the egg using muscular and chemical cues.
• Acrosome reaction: Sperm undergo enzymatic changes that allow them to penetrate the egg’s outer layer.
• Fusion of gametes: The nuclei of the sperm and egg merge to form a zygote.
• Cell division and implantation: The zygote divides and implants in the uterus, initiating foetal development.

Why Do Humans Practice Internal Fertilisation?

There are several biological and evolutionary reasons why humans and most other mammals rely on internal fertilisation:

• Protection from the environment: The female reproductive system provides a moist, controlled environment, which is essential for sperm survival.
• Increased chances of fertilisation: Sperm are not exposed to the external environment, enhancing the likelihood of successful fertilisation.
• Protection of the embryo: The embryo is shielded from predators, temperature fluctuations and other environmental hazards.
• Parental investment: Mammals with long gestation periods adopt reproductive strategies that maximise offspring survival.

Why Can Humans Not Naturally Use External Fertilisation?

In humans, fertilisation outside the body does not occur naturally. Sperm and eggs are highly sensitive to temperature, pH and pathogens. Outside the female reproductive tract, sperm lose motility and eggs risk dehydration and microbial damage.

Unlike aquatic organisms, humans have no external environment that can support gamete survival. Internal fertilisation evolved in terrestrial mammals due to these environmental limitations.

How Does IVF Mimic External Fertilisation?

In vitro fertilisation (IVF) is the only instance of human fertilisation outside the body. Eggs are retrieved from the ovaries and combined with sperm in a laboratory dish, where fertilisation occurs. The embryos develop for several days before being implanted into the uterus. While IVF occurs outside the body, it differs from true external fertilisation. The laboratory environment replicates aquatic conditions using controlled temperature, nutrient media and sterile conditions. According to the HFEA, these conditions are essential for preserving the embryo's viability.

Why Is Internal Development Still Essential in Humans?

Even if fertilisation occurs in a laboratory, human embryos cannot develop outside the body beyond the early stages. Implantation in the uterine lining is essential for a successful pregnancy, allowing the placenta to provide nutrients, oxygen and waste removal.

Medical research confirms that the uterus offers a unique environment that cannot be replicated externally. This biological need highlights an important point to consider when we differentiate between internal and external fertilisation in humans.

How Internal Fertilisation Enhances Survival in Humans?

The mode of human reproduction emphasises quality over quantity. Internal fertilisation provides the following advantages:

• Increased chances of fertilisation.
• Safety of the embryo from predators and diseases.
• Regulation of hormones in the developing embryo.

Because human beings usually give birth to only one offspring at a time, survival is associated with extensive prenatal care rather than mass production. This is in contrast to other species that use external fertilisation, where the massive gamete output compensates for high mortality rates.

What Role Does the Uterus Play After Fertilisation?

After implantation, the uterus remains the primary site of development for about 9 months. During this time:

• The placenta is responsible for regulating nutrient and oxygen supply.
• The amniotic fluid protects the fetus.
• The maternal hormones are responsible for growth and immune tolerance.

This internal developmental system is critical for the formation of complex organs, especially the human brain.

How Does Evolution Explain The Difference Between Internal & External Fertilisation?

External fertilisation was successful in water but not on land, where conditions would have been drier and more unpredictable. The mammals developed a reproductive system that favoured internal fertilisation, extended gestation periods and parental care.

This explains why humans are fully dependent on internal fertilisation and why it is important to distinguish between internal fertilisation and external fertilisation in evolutionary biology. Knowledge of what makes human reproduction special compared to external fertilisation assists in understanding:

• Fertility and infertility treatments.
• Contraceptives.
• Complications during pregnancy, etc.

It also explains why medical technology, such as IVF, enables fertilisation but not internal gestation. All of these are crucial to reproductive biology and medicine and they enable students to effectively differentiate between internal and external fertilisation.

Final Thoughts

The human reproductive system is built around internal fertilisation, providing a safe and precisely regulated environment for embryo development. Understanding this distinction helps us appreciate why humans cannot reproduce externally like many aquatic species and why treatments such as IVF, while powerful, still rely on internal development. Recognising this can empower you to make informed decisions about fertility, value the role of medical interventions and approach reproductive health with confidence and awareness.