Sexual reproduction is achieved through the process of fertilization i.e., the fusing of cell nuclei from a male gamete (sperm) and a female gamete (egg) to produce a zygote which through various stages of development produces an adult. Fertilization is achieved through one of two methods:
Internal fertilization - The fusion of the male and female gametes occurs within the female’s reproductive tract either through copulation (mammals) or [cloacal] fertilization (examples include [caecilians], most [salamanders] and many reptiles).
External fertilization – The fusion of the male and female gametes occurs outside of the animals’ body usually in a watery environment. These animals spawn or release their gametes into the water and by random chance the gametes join and fuse. [Anurans] and egg-laying fish are examples of animals that reproduce in this fashion.