Zygote: The Fertilised Egg Cell

The initial stage following the fusion of a male gamete (sperm) and a female gamete (egg) results in a single, fertilised cell. This first diploid cell marks the beginning of a new organism's development.

Defining Key Developmental Terms

Let's clarify the terms provided:

• Zygote: The immediate product of fertilization; a single-celled, fertilised egg.
• Embryo: A stage following the zygote, characterised by rapid cell division (cleavage) and differentiation, forming multiple cells and tissues.
• Foetus: A later developmental stage, typically after the embryonic period, where the organism has a more recognisably developed form.
• Blastocyst: An early-stage embryo consisting of about 100-200 cells, forming a hollow ball before implantation.

Identifying the Correct Term

The question specifically asks for the name of a "single-celled fertilised egg". Based on the definitions:

• The zygote perfectly matches the description of a single-celled fertilised egg.
• Embryo, foetus, and blastocyst represent later, multicellular stages of development.

Therefore, the correct term is zygote.