About Embryonic development
All events which change a zygote or a blastos (a reproductive unit in asexual reproduction e.g., whole organism e.g. Amoeba, a bud e.g. Hydra, or a body fragment e.g. sea anemone) into a fully developed organism is called development. The entire process of development, which passes through embryo formation is called embryogenesis. Von Baer is commonly called “The father of modern embryology.”To excel in your Biology exam must refer NCERT text book and solve the questions given in exercise of NCERT for reference you can use Physics Wallah NCERT solutions for class 12 biology . Prepared by Physics Wallah experts
Type of Embryonic development
(i) Pre natal or embryonic period : It is the period of development from the diploid one-celled zygote to a multicellular embryo. It occurs either inside the egg or mother’s womb and extends upto hatching or birth. The study of the changes during this period is called embryology.
(ii) Post natal or post embryonic period : It is the period of development which extend from hatching or birth to death. The branch of science which deals with the study of progressive, orderly and gradual changes in structure and functioning of organism during entire life history from zygote or blastos to death, is called development biology.
Phases of embryonic development
Embryonic development involves following dynamic changes and identifiable process.
(i) Gametogenesis : It involve the formation of haploid sex cells or gametes called sperms and ova from diploid primary germ cells called gametogonia present in the reproductive organs called gonads (testes and ovary). It is of two types
(a) Spermatogenesis : Formation of sperm. (b) Oogenesis : Formation of ova
(ii) Fertilization : It involve 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 chromosomes of two gametes is called amphimixis.
(iii) Cleavage : It includes the rapid mitotic division of the zygote to form a single layered hollow spherical larva called blastula and its formation is called blastulation.
(iv) Implantation : The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called implantation.
(v) Gastrulation : It includes the mass and orderly migration of the organ specific areas from the surface of blastula to their predetermined position which finally produces a 3 layered gastrula larva. It is with 3 primary layers.
(vi) Organogenesis : It includes the formation of specific organs system from three primary germ layers of gastrula and also includes the morphogenesis and differentiation.