Step 1: Understanding the Concept:
Following successful fertilization in the ampullary region of the fallopian tube, the single-celled embryo undergoes a series of rapid mitotic divisions called cleavage. As the embryo travels down toward the uterus, it transforms through distinct structural embryonic stages before attaching to the maternal uterine wall.
Step 2: Detailed Explanation:
Let's trace the early development steps in human reproduction chronologically:
1. Formation of Zygote (A): The fusion of a haploid sperm and a haploid ovum produces a single diploid cell called a zygote. This marks day 1 of development.
2. Formation of Morula (D): The zygote moves through the fallopian tube and undergoes cleavage, dividing into 2, 4, 8, and then 16 daughter cells called blastomeres. The solid, mulberry-like sphere containing 8 to 16 blastomeres is called a morula. This stage occurs around days 3 to 4.
3. Formation of Blastocyst (B): As the morula enters the uterus, fluid accumulates inside, creating a hollow cavity called the blastocoel. The cells rearrange into an outer layer (trophoblast) and an inner cell mass. This hollow structure is called a blastocyst (days 5 to 6).
4. Implantation (C): Around days 7 to 9, the blastocyst embeds itself securely into the vascularized inner lining of the uterus (endometrium).
Arranging these steps in chronological order gives: A $\rightarrow$ D $\rightarrow$ B $\rightarrow$ C. This matches option (A).
Step 3: Final Answer:
The correct chronological sequence of events is A, D, B, C.