The correct option is (a).
Explanation
The development of a new individual in sexual reproduction follows a specific biological sequence starting from fertilization and leading to the formation of a fully developed organism. This process involves cell division, differentiation, and organogenesis.
Chronological Sequence of Development:
- Zygote (2): This is the first stage of development. It is a single cell formed by the fusion of a male gamete (sperm) and a female gamete (ovum) during fertilization.
- Ball of cells (4): The zygote undergoes repeated mitotic divisions (cleavage) as it moves down the oviduct. This results in a solid ball of cells known as the morula, which eventually develops into a hollow structure called the blastocyst.
- Embryo (3): Once the ball of cells (blastocyst) implants itself into the wall of the uterus, it is referred to as an embryo. During this stage, cells begin to differentiate into specific tissues and organs.
- Foetus (1): This is the stage of the embryo in which all the main body parts (such as hands, legs, head, eyes, etc.) can be identified. In humans, the embryo is generally termed a foetus from the eighth week of pregnancy until birth.
Therefore, the correct sequence is Zygote → Ball of cells → Embryo → Foetus (2 - 4 - 3 - 1).
Key Takeaway: Development proceeds from a single-celled zygote to a multicellular cluster (ball of cells), followed by tissue differentiation (embryo), and finally the maturation of identifiable body structures (foetus).