Internal vs External Fertilization & Development

Fertilization Fertilization is the union of egg and sperm Egg (n) + Sperm (n) = zygote 2n

External Fertilization External fertilization occurs outside of female parent (joining of egg and sperm outside female reproductive tract) Fish Amphibians Aquatic vertebrates

External Fertilization Produce MANY eggs in water because: Harsh environmental conditions Predators Chance of fertilization is low

Internal Fertilization Internal fertilization occurs inside the female reproductive tract * mammals, reptiles, birds

Internal Fertilization Produce few eggs - Chance of survival is high Zygote forms inside the female body.

Development of the fertilized egg External Development Develops outside the parent ** fish, frogs, and any other egg laying organisms.

Internal Development Internal Development occurs in inside female Placental mammals -that possess an internal structure that forms joining of embryo and mother (mice, cats dogs, whale, human, etc.)

So here are the possible situations Internal Fert + Internal Dev = Mammals Internal Fert + External Dev = birds, reptiles (egg layers) External Fert + External Dev = fish, frogs