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What is the process of implantation? OR

Where does fertilization take place in the female reproductive system? OR

How does a sperm penetrate an egg during fertilization? OR

How does the female gamete move from the ovary to the uterus?


  1. The process of fertilization initiates when the male deposits semen in the female vaginal tract through penis.
  2. Nearly 3 to 4 ml of semen contains billions of the sperms.
  3. As the semen deposited in the vaginal tract, presented sperms start moving towards the oviduct through uterus.
  4. For this locomotion, contraction of vaginal passage and uterine wall helps, as well as the slimy secretion of oviduct wall helpful for the movement of sperms.
  5. This whole process takes 5 to 6 hours approximately.
  6. After the successful locomotion of sperms, they come in the contact of secondary oocyte that is surrounded by egg membrane and a jelly layer.
  7. Now on the acrosome of sperms various enzymes are present, among them hyaluronidase enzyme helps the sperm to penetrate inside the oocyte.
  8. As the head of sperm enters the secondary oocyte, the nucleus of head enlarges into male pronucleus.
  9. This entry of sperm stimulates some immediate changes, such as egg membrane becomes slightly separated from protoplasm known as fertilization membrane and also induces the completion of maturation of secondary oocyte by forming female pronucleus.
  10. The fertilization membrane prevents other sperms to enter into oocyte.
  11. Here, by the fusion of male pronucleus and female pronucleus forms diploid zygote nucleus, the ovum is now called as zygote.
  12. The formed zygote passes through the process of embryo development and develops an infant.


  1. The developmental process of embryo takes first 2 months, and then a foetus is formed.
  2. The rest of the developmental period is known as fetal development.
  3. As the fertilization takes place in the oviduct near the ovary, the cleavage occurs, due to that zygote undergoes the cell division but the size of the cell remains the same, i.e. size doesn’t increase.
  4. During this process of cleavage, the developing embryo moves towards the uterus and the formation of blastomere occurs.
  5. As the embryo reaches the uterus, it forms 16-celled structure which is known as morula. This formed at the third day of fertilization.
  6. By the fifth day, morula is converted into a blastocyst that is a fluid-filled bag like structure.
  7. On the outer side of blastocyte, a thin single layer of cell occurs, known as trophoblast.
  8. The mass of cell present in the trophoblast is known as inner cell mass that is responsible for the formation of the embryo.
  9. The fluid present in the blastocyte in due to the secretion from trophoblast.
  10. Later the trophoblast associate with the layer of mesoderm and form an extra-embryonic membrane called “CHORION”.
  11. All these changes take place in a week period of time.
  12. Now the process of embryo implantation takes place in the uterine wall.
  13. As the initial stage of the implantation, the jelly like layer around the embryo is removed.
  14. The fluid in the blastocyte, secreted by the trophoblasts contains the enzymes that digest some tissues and blood vessels in the uterus.
  15. This digestion helps in the implantation of embryo in the uterine wall by the development of inner wall and partially development of embryo. This process is called embryo implantation.
  16. At this stage of implantation, trophoblast secretes a hormone that is essential for the verification of the implantation.
  17. Now the inner mass of the wall is separated from trophoblast, meanwhile extra-embryonic membrane development begins.
  18. In the second week of embryo development, the inner mass is converted into a flat plate like structure that is known as “embryonic disc” and due to that the process of gastrulation occurs.
  19.  There is formation of two layers in the embryonic disc: the upper layer constitutes ectoderm and the inner layer constitutes endoderm
  20. Now the embryonic disc elongates and forms a primitive streak and through the invagination of this region, the formation of mesoderm takes place.


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