Arrange the following events in the correct order pertaining to fertilization in the human reproductive system

(A) The blastocyst becomes embedded in the endometrium.

(B) Finger-like projections appear on the trophoblast called chorionic villi.

(C) The blastomeres are arranged into trophoblast and the inner cell mass.

(D) The zygote divides mitotically and transforms into an embryo with 8-16 blastomeres, called a morula

The correct option is (D), (C), (A), (B).

Let us understand the step-by-step sequence of events following fertilization in the human reproductive system:
1. Formation of the Morula (Event D): After fertilization, the single-celled zygote undergoes rapid mitotic divisions called cleavage. As it moves through the fallopian tube towards the uterus, it divides into 2, 4, 8, and then 16 cells. These individual cells are called blastomeres. The solid embryo with 8 to 16 blastomeres is shaped like a mulberry and is called a morula.
2. Formation of the Blastocyst (Event C): As the morula enters the uterus, it continues to divide and transforms into a hollow ball of cells called a blastocyst. At this stage, the blastomeres organize into two distinct regions: an outer layer of cells called the trophoblast and an inner cluster of cells called the inner cell mass.
3. Implantation (Event A): The blastocyst then prepares to attach to the uterine wall. The trophoblast layer attaches to the endometrium (the inner lining of the uterus). Following this attachment, the uterine cells divide rapidly and cover the blastocyst, causing it to become completely embedded in the endometrium. This process is called implantation and marks the beginning of pregnancy.
4. Development of Chorionic Villi (Event B): After implantation, finger-like projections called chorionic villi appear on the trophoblast. These villi are surrounded by uterine tissue and maternal blood, eventually helping to form the placenta which supports the developing embryo.

Thus, coordinating these stages in chronological order gives the sequence: (D) → (C) → (A) → (B).