What is the difference between external and internal fertilization?

Fertilization is fusion of male gamete sperm with female gamete egg. This may take place within reproductive tract of female organism, or outside in environment.

External fertilization takes place in both invertebrates and vertebrates like (bony fishes and amphibians) where both sperms and eggs are released in the watery environment and gametes fuse by chance encounter. Such external fertilization may be highly modified as seen in earthworms where eggs are laid in a cocoon, then sperms are collected from another earthworm and later deposited inside the cocoon containing eggs.

Internal fertilization occurs by transfer of sperm in female body (insemination) and fertilization takes place in vivo. Internal fertilization takes place in insects, in nematode worm Wuchereria bancrofti, in cartilagenous fishes as well as in amniotes (reptiles, birds, mammals). For internal fertilisation, organisms must copulate. Chance of fertilization of gametes is very high when it takes place in vivo.