Describe the developmental stages of human embryo after fertilization.
1) During fertilization, chromosomes of the ovum and the chromosomes of the sperm make up into pairs and the resulting cell is called zygote.
2) Fertilization takes place in the oviduct or fallopian tube.
3) The zygote which is diploid travels down the fallopian tube. As it moves it undergoes several mitotic divisions forming the embryonic stage called blastocyst.
4) Blastocyst moves towards the wall of the uterus and finally gets attached and embedded in the wall of the uterus. This is called implantation.
5) The growing embryo forms two membranes chorion and Amnion.
6) Chorion establishes connection with the walls of the uterus and helps in the supply of nutrients to the embryo and removal of wastes from the embryo.
7) Amnion forms a sac like structure around the embryo. The space between the amnion and embryo is filled with a fluid called amniofic fluid.
8) Amnion and amniotic fluid give protection to the embryo against minor mechanical injury.
9) Placenta is a tissue formed by the cells from the embryo and the mother. It is formed around 12 weeks of pregnancy.
10) Placenta nourishes the growing embryo.
11) A tough cord called umbilical cord is also formed by the embryo which is connected to the walls of the uterus through the placenta.
12) From 3 months of pregnancy, the embryo is called foetus.
13) Pregnancy lasts on an average 9 months or 280 days. This period is called gestation period.