X and Y are interdigitated to form placenta. In the above statement X and Y are

	X	Y
1.	Chorionic villi	Uterine tissue
2.	Uterine tissue	Maternal blood
3.	Inner cell mass	Chorionic villi
4.	Chorionic villi	Umbilical cord

The placenta is formed by the interdigitation of chorionic villi, which are finger-like projections from the trophoblast of the developing embryo (foetus), with the maternal uterine tissue (endometrium). This interdigitation creates a structural and functional unit that connects the developing fetus with the mother's body.

The placenta plays a vital role during pregnancy as it serves as the interface for the exchange of gases, nutrients, and waste products between the fetal and maternal circulatory systems. It ensures that the developing foetus receives the necessary oxygen and nutrients while also removing waste products. This exchange occurs through the thin walls of the chorionic villi, which are bathed in maternal blood.

The interdigitation and close contact between the chorionic villi and the uterine tissue facilitate this exchange while also providing a barrier to prevent direct mixing of the two circulatory systems, ensuring the health and development of the fetus.