It divides repeatedly to form a ball of cells.
Explanation
In human sexual reproduction, fertilization results in the formation of a single-celled diploid structure called the zygote. This cell contains the genetic material from both parents and serves as the starting point for the development of a new organism.Analysis of Options:
- It is expelled from the body during menstruation. is Incorrect: Menstruation occurs only if fertilization does not take place. In that scenario, the unfertilized egg and the thickened lining of the uterus are shed. If a zygote is formed, hormonal signals prevent menstruation to sustain the pregnancy.
- It divides repeatedly to form a ball of cells. is Correct: Immediately after formation, the zygote moves through the oviduct (fallopian tube) and undergoes a series of rapid mitotic divisions known as cleavage. This process transforms the single-celled zygote into a solid ball of cells (morula) and subsequently a hollow ball of cells (blastocyst), which eventually implants in the uterus.
- It immediately differentiates into a fully formed foetus. is Incorrect: Development is a gradual process. The zygote first develops into an embryo. The term "foetus" is technically applied only after the eighth week of development, once the major body structures and organs have formed. Immediate differentiation into a fully formed foetus does not occur.
- It undergoes binary fission to produce two independent zygotes. is Incorrect: Binary fission is a method of asexual reproduction seen in unicellular organisms like Amoeba and bacteria, where one cell divides into two independent individuals. Human zygotes undergo mitosis to build a multicellular organism, not to create independent zygotes.
Key Takeaway: The zygote is the first cell of a new individual which, immediately upon formation, undergoes repeated mitotic divisions to form a multicellular embryo capable of implantation.