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Developmental biology or embryology is that branch of Biology which deals with the development of animals.
So long as the developing animal is inside the egg, it is called as embryo.
Therefore, the term Embryogenesis or Embryogeny may be defined as formation and development of embryos. It includes all the changes by which a fertilized egg is transformed into an adult.
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In some lower animals the amount of nutritive material in egg is less and insufficient to support the embryonic development. So the embryo hatches in an earlier stage of development, called a larva (example-tadpole larva of frog) and this type of development is called indirect development. When the amount of nutritive material is sufficient, the embryo hatches into an infant and this type of development is called direct development.
Major Events of Development:
1. Gametogenesis:
It includes the formation of male gametes or sperms by spermatogenesis and female gametes or ova by oogenesis.
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2. Fertilization:
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This is union of sperm and ovum to form the zygote.
3. Cleavage:
It is repeated mitotic division of the zygote into a piulticellular structure called morula or blastula.
4. Gastrulation:
Due to cellular movements and arrangements, the blastula is converted into a two germinal layered or three germinal layered gastrula.
5. Organogenesis:
The germ layers give rise to formation of organs.
6. Growth and differentiation:
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The organs grow in size and the cells of the organs change in structure and function.
7. Metamorphosis:
It is the process of transforming the larva into adult
8. Regeneration:
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It includes the repair of lost parts of the adult.
