Mammals have internal gestation with viviparous birth.
Cheetahs are placental mammals, so they have internal fertilization and internal development within the uterus of the female.
its internal
polar bears are internal because they are mammals and all mammals are internal.
Most of the fishes, All amphibians, reptiles, birds and a very few mammals have external development.
Internal - they're mammals.
Opossums are marsupial mammals - they have internal fertilization as all mammals do.
Intertnal --
internal
mammals
External
It is internal because spider monkeys are mammals.