Embryogenesis

Embryogenesis

Embryogenesis is the process by which the zygote develops into an embryo. It has two components working together: cell division (by mitosis), which increases the number of cells in the developing embryo, and cell differentiation, by which groups of these cells become specialised into distinct tissues and organs that together build the body of the new organism.

Oviparous and Viviparous Animals

Animals are classified according to where the zygote completes its development:

• Oviparous animals — such as reptiles and birds — lay fertilised eggs, protected by a hard calcareous shell, in a safe location in the environment; the young hatch out of the shell after a period of incubation outside the mother's body.
• Viviparous animals — the majority of mammals, including humans — retain the developing zygote inside the mother's body, where it grows into a young individual that is only delivered once it has reached an advanced stage of development.

Because viviparous young receive continuous embryonic care and protection inside the parent's body, their survival chances are generally much higher than those of oviparous young, whose eggs and hatchlings are exposed to the environment and to predators from an early stage.

Post-fertilisation Changes in Flowering Plants …