Explain the events that take place once a sperm reaches the oviduct till it becomes a foetus. Write the role of placenta in pregnancy.

1. Events from Sperm reaching Oviduct to Foetus:

• Fertilisation: The sperm reaches the oviduct (fallopian tube) where it encounters the mature egg cell (ovum). The fusion of the male gamete (sperm) and female gamete (egg) occurs here to form a single-celled diploid zygote.
• Cleavage and Embryo Formation: The zygote begins to divide repeatedly by mitosis as it travels down the oviduct, forming a hollow ball of cells called the embryo.
• Implantation: The embryo reaches the uterus and gets embedded (implanted) in its thick, soft, and highly vascularized inner lining (endometrium).
• Development into Foetus: The implanted embryo continues to grow and differentiate. Cells specialize to form tissues and organs. When the body parts of the developing organism become clearly identifiable, it is called a foetus.

2. Role of the Placenta in Pregnancy:

• Nutrition: It provides a large surface area (via villi) for the transfer of glucose, amino acids, and other essential nutrients from the mother's blood to the developing embryo.
• Respiration: It facilitates the supply of oxygen from the mother to the embryo and the removal of carbon dioxide from the embryo.
• Excretion: It helps in disposing of metabolic nitrogenous wastes generated by the growing embryo by transferring them into the mother's blood stream for excretion.