1. Internal vs External Fertilization & Development

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

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

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

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

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

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

8. 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.)

9. 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