(a) Explain the events taking place at the time of fertilization of an ovum in a human female.
(b) Trace the development of the zygote upto the implanation in the uterus.
(c) Name and draw a labelled sectional view of the embryonic stage that gets implanted.

Explanation

(a) Fertilisation :
(i) The process of fusion of a sperm (male gamete) with an ovum (female gamete)is called fertilisation.

(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 vagina (insemination).
(iv) The motile sperms swim rapidly, through the cervix, into the uterus and reach the ampullary- isthmic junction of the oviduct.
(v) A sperm comes in contact with the zona pellucide layer of the ovum and induces changes in the membrane to block the entry of additional sperms.
(vi) 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. ffitimately, diploid zygote is produced by the fusion of a sperm and an ovum.
(b) Implantation : (i) Zygote divides rapidly by mitiotic 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 cell of endometrium divide rapidly and cover the blastocyst.
(vii) The blastocyst gets embedded in the endometrium of the uterus called implantation of pregnancy.
(viii) The Embryonic stage that gets implanted is called blastocyst