Pregnancy and Embryonic Development – Class 12 | Chapter – 3 | Biology Short Notes Series PDF

Pregnancy and Embryonic Development: The different phases of development of an embryo that takes place inside a mother’s womb or a flower are together called embryonic development. According to the embryonic development definition, a unicellular zygote converts into an embryo due to cell division inside the protective cover. One cell will divide to become 2 in this cell division. It will then form 4 cells that eventually become 8. This is how rapidly cell division occurs resulting in the formation of an embryo.

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The different phases of development of an embryo that takes place inside a mother’s womb or a flower are together called embryonic development. According to the embryonic development definition, a unicellular zygote converts into an embryo due to cell division inside the protective cover. One cell will divide to become 2 in this cell division. It will then form 4 cells that eventually become 8. This is how rapidly cell division occurs resulting in the formation of an embryo.

Pregnancy and Embryonic Development

Weeks after Fertilization Embryonic Development
Week Three Gastrulation and formation of three germinal layers. Neurulation follows gastrulation. Notochord is formed
Week Four The heart is the first organ to start functioning. The heart starts beating. Arm buds and optic pits become visible
Week Five Size ~ 4 mm. Starts becoming C-shape, inner ear starts developing, pharyngeal arches develop, liver, pancreas, spleen and gall bladder start developing
Week Six Size ~ 8 mm. Development of eyes and nose, leg buds and hand as flat paddles appear, stomach and kidney precursors start developing
Week Seven Size ~ 13 mm. Lungs and lymphatic system and primary sex organs start developing, arms and legs lengthen and digits start appearing
Week Eight Size ~ 20 mm. External ear starts appearing, nipples and hair follicles start developing, most of the organs start developing by this time

Embryogenesis

Formation of embryo from zygote includes following stages :

  • Cleavage
  • Blastulation
  • Gastrulation
  • Foetal development

 


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