Class 12 Biology Reproduction In Organisms Post- fertilization events

Post- fertilization events

Zygote formation

  • In some organisms like fungi and algae, the zygote develops a thick wall around it which is resistant to damage and undergoes a period before germination.
  • Further development of zygote depends on the type of life cycle of organisms and the environment in which they are surviving.
  • In all sexually reproducing organisms, a diploid zygote is formed as a result of fertilization and later the zygote develops into a new organism.


  • The process of development of embryo from the zygote is called embryogenesis.
  • Embryogenesis includes cell division and cell differentiation in the zygote.
  • Cell division increases the number of cells and cell differentiation helps to form tissues and organs to form a fully developed organism.

Fig. embryogenesis

  •  Depending on the development of zygote inside or outside the body of the female parent, animals are classified into oviparous and viviparous animals.
  • Oviparous animals are those which lay eggs and the eggs develop into new organisms. Example- amphibians, fishes, birds, reptiles etc.
  • Viviparous animals are those which give birth to their young ones. Example- mammals.
  • In some oviparous animals like reptiles and birds the fertilized eggs remain covered by hard calcareous shell and such eggs are called cleidoic
  • In flowering plants, the zygote is formed inside the ovule and the sepals, petals, stamens of the flower usually fall off.
  • The zygote develops into embryo
  • Ovules develop into seeds
  • The ovary develops into the fruit.
  • The ovary wall develops into a protective layer called pericarp.

Fig. fruit and seed formation

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