Zygote:

A zygote is the earliest stage of human development, formed by the fusion of a sperm cell and an egg cell during fertilization. It is a single-celled structure that contains all the genetic information necessary to develop into a complete human being.

Formation:
- The formation of a zygote occurs when a sperm cell penetrates the egg cell, leading to the fusion of their genetic material.
- This process typically takes place in the fallopian tube, shortly after sexual intercourse or artificial insemination.

Characteristics:
- The zygote is a diploid cell, meaning it contains two sets of chromosomes (one set from each parent). This ensures the zygote has the necessary genetic information to develop into a fully formed individual.
- It is the initial stage of embryonic development and marks the beginning of prenatal development.

Development:
- After fertilization, the zygote undergoes a series of cell divisions called cleavage. These divisions result in the formation of a multicellular structure known as the embryo.
- The zygote travels down the fallopian tube and implants itself into the uterine wall, where it receives nourishment from the mother's body.

Foetus:

A foetus is the later stage of prenatal development, following the embryonic stage. It begins around the ninth week after fertilization and continues until birth.

Development:
- During the foetal stage, the embryo undergoes significant growth and development. Organs, tissues, and body systems continue to develop and mature.
- By the end of the third month, the foetus has formed identifiable features such as limbs, fingers, toes, and facial structures.
- The foetus also develops a circulatory system, nervous system, digestive system, and other vital organs. It grows in size and becomes more active, with movements becoming noticeable to the mother.

Viability:
- The foetus becomes viable, meaning it has the potential to survive outside the womb, around the 24th week of pregnancy.
- At this stage, if born prematurely, the foetus may require medical intervention and specialized care in a neonatal intensive care unit (NICU) to support its continued development.

Conclusion:
In summary, a zygote is the initial stage of development following fertilization, consisting of a single cell with the full genetic information. On the other hand, a foetus is a later stage of development characterized by significant growth, organ formation, and increased viability. Understanding the differences between these stages helps in comprehending the progression of human development and the various milestones reached during pregnancy.