Sexual Reproduction in Animals - Post-fertilisation Events in Organisms

Post-fertilisation Events in Organisms:

• Events in sexual reproduction after the formation of the zygote are called post-fertilization events.

A) The Zygote:

• The 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.
• In organisms belonging to fungi and algae, the zygote develops a thick wall that is resistant to desiccation and damage. It undergoes a period of rest before germination. 
• In organisms with a haplontic life cycle, the zygote divides by meiosis to form haploid spores that grow into haploid individuals. 
• Every sexually reproducing organism, including human beings begin life as a single cell - the zygote.
  

  The Zygote

B) Embryogenesis:

•  Embryogenesis refers to the process of development of the embryo from the zygote.
• During embryogenesis, the zygote undergoes cell division (mitosis) and cell differentiation. While cell divisions increase 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.

Embryogenesis in animals:

Animals are categorised based on whether the development of the zygote takes place outside the body of the female parent or inside.

1. Oviparous animals: Development of zygote takes place outside the body of organisms and lays fertilised/ unfertilized eggs. In oviparous animals like reptiles and birds, the fertilised eggs covered by the hard calcareous shells are laid in a safe place in the environment; after a period of incubation young ones hatch out.
   
   
2. Viviparous animals: Development of the zygote takes place within the uterus of the female parent; the offspring are born as a juvenile. The embryo in viviparous organisms receives nourishment from the mother’s blood through the placenta. After attaining a certain stage of growth, the young ones are delivered out of the body of the female organism. Because of proper embryonic care and protection, the chances of survival of young ones are greater in viviparous organisms.
   

Embryogenesis in flowering plants:

In flowering plants, the zygote is formed inside the ovule. After fertilization, the sepals and stamens of the flower wither and fall off. The pistil, however, remains attached to the plant. The zygote develops into the embryo and the ovules develop into the seed. The ovary develops into fruits which develop a thick wall called the pericarp, protective in function. After dispersal, seeds germinate under favourable conditions to produce new plants.