12th Class Biology Human Reproduction Phases Of Embryonic Development

Embryonic development involves following dynamic changes and identifiable process.

(1) 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

(i) Spermatogenesis : Formation of sperm.

(ii) Oogenesis : Formation of ova

(2) 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.

(3) 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.

(4) Implantation : The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called implantation.

(5) 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.

(6) Organogenesis : It includes the formation of specific organs system from three primary germ layers of gastrula and also includes the morphogenesis and differentiation.

Important Tips

Historical background of Embryonic Development:

• George Newport: Observed fertilization of frog’s egg.
• Oscar Hertwig: Described the fusion of sperm and egg nuclei in sea urchin.
• Prevost and Dumas: Reported cleavage of frog’s egg.
• Swammerdam: Observed the first cleavage of frog in 1738.
• Spallanzani: Detailed process of cleavage of frog’s egg.
• Von Bear: Proposed recapitulation theory.
• Ernst Haeckel: Modified recapitulation theory to “Biogenetic law”. It states “Ontogeny repeats phylogeny.”
• Spemann and Mangold: Reported embryonic induction on newt and gave concept of primary organizers.
• Pander: Formation of three germinal layers in chick embryo.
• Oviparity: Fertilization may be external or internal but development always outside the female body and inside the egg e.g. most of non-chordates, fishes, amphibians and reptiles; all birds and prototherians.
• Ovoviviparity: Fetilization always internal. Development also inside the uterus and baby is born but there is no placenta formation so egg is yolky e.g. rattele snake, Dog fish
• Viviparity: Fertilization and development always inside the body. Placenta is formed and female gives birth to young one e.g. most of mammals.