A developing structure where cells differentiate into tissues and organs.
Explanation
In the biological process of sexual reproduction, development proceeds through distinct stages. Following fertilization, the organism transitions from a single cell to a complex multicellular structure through division and differentiation.
Detailed Analysis:
- A single cell resulting from the fusion of gametes. is incorrect: A single cell resulting from the fusion of male and female gametes is called a zygote. This is the immediate product of fertilization and is unicellular.
- A developing structure where cells differentiate into tissues and organs. is correct: The embryo is the early developmental stage of a multicellular organism that follows the zygote. During this phase, the zygote undergoes repeated cell division (mitosis) and the resulting cells begin to differentiate into specific tissues and organs.
- A fully developed offspring ready for birth. is incorrect: A developing offspring that has acquired the distinct features of the species and is approaching birth is typically referred to as a foetus (in mammals). The embryonic stage precedes the foetal stage.
- An unfertilised egg released from the ovary. is incorrect: An unfertilised egg released from the ovary is a female gamete, known as an ovum. It is a haploid cell that has not yet undergone development.
Key Takeaway:
The embryo represents the stage of development characterized by rapid cell division and the differentiation of cells into tissues and organs, bridging the gap between the unicellular zygote and the more advanced foetus.