Post-fertilisation events

 The events in sexual reproduction which occurs after the formation of zygote are called post-fertilisation events.

The Zygote

Formation of the diploid zygote is universal in all sexually reproducing organisms.

In organisms with external fertilisation, zygote is formed in the external medium (usually water),

whereas in those exhibiting internal fertilisation, zygote is formed inside the body of the organism.

Further development of the zygote depends on the type of life cycle

the organism has and the environment it is exposed to.

In organisms belonging to fungi and algae, zygote develops a thick wall that is resistant

to dessication(extreme dryness) and damage and undergoes a period of rest before germination.

Zygote is the vital link that make sure the continuity of species between organisms of one generation and the next.

Every sexually reproducing organism, including human beings begin life as a single cell known as the zygote.

Embryogenesis The process of development of embryo from the zygote is known as embryogenesis.

During embryogenesis, zygote undergoes cell division (mitosis) and cell differentiation.

Cell divisions increases the number of cells in the developing embryo;

cell differentiation helps groups of cells to undergo certain modifications to form specialised tissues and organs to form an organism.

Based on the development of the zygote which takes place either outside the body of the

female parent or inside animals are categorized into oviparous and viviparous ,

i.e., whether they lay fertilised/unfertilised eggs or give birth to young ones.

In oviparous animals like reptiles and birds,

the fertilised eggs covered by hard calcareous shell are laid in a safe

place in the environment; after a period of incubation young ones comes out.

Whereas, in viviparous animals i.e majority of mammals including human beings,

the zygote develops into a young one inside the body of the female organism.

After certain stage of growth,

the young ones are delivered out of the body of the female organism.

Due to proper care and protection, the chances of survival of young ones is greater in viviparous organisms.

In flowering plants, the zygote is formed inside the ovule.

After fertilisation the sepals, petals and stamens of the flower wither and fall off.