Direct vs. Indirect Fertilisation:

Direct fertilisation occurs when the egg and sperm come into contact within the female reproductive system. This is common in most mammals, including humans. Indirect fertilisation, on the other hand, involves the release of gametes into the environment where fertilisation takes place externally, as seen in many aquatic species.

Direct Fertilisation in Humans:
In humans, direct fertilisation occurs internally within the female reproductive tract. During sexual intercourse, sperm is deposited into the vagina and travels through the cervix and uterus to reach the fallopian tubes where fertilisation of the egg usually takes place.

Indirect Fertilisation:
Although indirect fertilisation is not seen in humans, it is common in many aquatic species such as fish, amphibians, and some invertebrates. In these species, eggs and sperm are released into the water where fertilisation occurs externally.

Key Differences:
- Direct fertilisation occurs internally in the female reproductive tract in humans, while indirect fertilisation occurs externally in the environment.
- In direct fertilisation, sperm and egg come into contact within the female reproductive system, while in indirect fertilisation, fertilisation takes place outside the body.
- Direct fertilisation is more common in terrestrial animals, while indirect fertilisation is common in aquatic species.

In conclusion, humans exhibit direct fertilisation where the egg and sperm meet internally within the female reproductive tract. This process is essential for the successful reproduction and development of offspring in mammals, including humans.