(i) Post-zygotic events upto implantation and placenta formation
(ii) During coitus, semen is released by the penis into the vagina (insemination).
(iii) The motile sperms swim rapidly, through the cervix, enter into the uterus and reach the ampullary–isthmic junction of the oviduct.
(iv) A sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane to block the entry of additional sperms.
(v) The enzymes of the acrosome of sperm help to dissolve zona pellucida and plasma membrane of the ovum and sperm head is allowed to enter into the cytoplasm of the ovum, i.e., secondary oocyte.
(vi) Ultimately diploid zygote is produced by the fusion of a sperm and an ovum.
Implantation
(i) Zygote divides rapidly by mitotic division called cleavage and as a result 2, 4, 8, 16 daughter cells are produced which are termed as blastomeres.
(ii) Embryo with 8–16 blastomeres is called a morula.
(iii) The morula changes into a large mass of cells called blastocyst which passes further into the uterus.
(iv) Blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called inner cell mass.
(v) The trophoblast layer gets attached to the cells of the endometrium and the inner cell mass gives rise to the embryo.
(vi) The cells of endometrium divide rapidly and cover the blastocyst.
(vii) So, the blastocyst gets embedded in the endometrium of the uterus called implantation leading to pregnancy.
Functions of Placenta
(i) Embryo get nutrients, oxygen and removes CO2 and waste materials through placenta.
(ii) An umbilical cord connects placenta with the embryo that helps in the transport of substances to and from the embryo.
(iii) Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progesterons that are essential to maintain pregnancy.