Embryonic developmentEmbryonic development (Embryogenesis) includes a definite series of phases which are fundamentally similar in all sexually reproducing organisms,and transform one-celled zygote to a multicellular and fully formed developmental stage till hatching or birth. Such a remarkable similarity of embryonic development proves that all metazoans are interrelated and have common ancestory.
Phases of Embryonic DevelopmentEmbryonic development involves five dynamic changes and identifiable processes:
1. Gametogenesis :
It involves the formation of haploid sex cells or gametes,called gametogonia present in the reproductive organs called gonads (testes and ovaries).Gametogenesis is of two types
(a) Spermatogenesis- formation of sperms.
(b) Oogenesis- formation of ova.
It includes the fusion of haploid male and female gametes to form diploid zygote. The fusion of gametic pronuclei is called karyogamy while the mixing of two sets of chromosome of two gametes is called amphimixis.
It includes the rapid mitotic division of the zygote to form a single layered hollow spherical larva called blastula. Blastula formation is called blastulation.