sexual: internal and external

• • Fertilisation is the process of the sperm fusing with the egg to produce a zygote (fertilised egg). Fertilisation can occur either externally or internally.
• • As animals moved from protective aquatic environments to exposed terrestrial environments, there was a need to shift from external fertilisation to internal fertilisation to prevent dehydration of gametes.
• • External fertilisation
  • fertilisation of the egg by sperm occurs outside the female’s body
  • suited to organisms in aquatic environments or very moist environments, e.g. coral (known as coral spawning), fish and amphibians
  • suited to organisms in aquatic environments or very moist environments, e.g. coral (known as coral spawning), fish and amphibians
• • Internal fertilisation
  • fertilisation of the egg by sperm occurs inside the female’s body
  • suited to organisms in terrestrial environments, e.g. some invertebrates, reptiles, birds and mammals and some aquatic mammals (whales, dolphins)
  • small numbers of gametes (egg & sperm) are produced as fewer fertilised eggs are required to ensure the success of offspring
  • maybe a fertilised egg that develops a shell and can be laid or continues to develop inside the female's body