Difference between Embryo, Foetus, Gamete, and Zygote

Embryo:
An embryo is the early stage of development in multicellular organisms, including humans, after fertilization and implantation. It refers to the developmental stage from the moment of fertilization until about the eighth week of gestation. During this period, the embryo undergoes rapid cell division and differentiation, giving rise to various organ systems and body structures. The embryo is the stage at which the basic body plan is established.

Foetus:
A foetus is the later stage of development in humans and other mammals, following the embryonic stage. It begins around the ninth week of gestation and continues until birth. During this period, the foetus undergoes further growth and development, with the organs and systems becoming more refined and functional. The foetus is characterized by the presence of recognizable features, such as limbs, facial features, and internal organs.

Gamete:
A gamete is a specialized reproductive cell that contains half the number of chromosomes found in somatic cells. These cells are involved in sexual reproduction and are responsible for combining with another gamete to form a zygote. In humans, the male gamete is called a sperm, while the female gamete is called an egg or ovum. Gametes are haploid cells, meaning they contain only one set of chromosomes, whereas somatic cells are diploid, containing two sets of chromosomes.

Zygote:
A zygote is the result of the fusion of two gametes during fertilization. It is the initial cell that forms at the moment of conception when a sperm fertilizes an egg. The zygote contains a complete set of chromosomes, with half contributed by the father and half by the mother. It represents the beginning of a new individual's development and is the first step in the formation of an embryo.

Summary:
- An embryo is the early stage of development in multicellular organisms after fertilization and implantation, lasting until about the eighth week of gestation.
- A foetus is the later stage of development in humans and mammals, starting around the ninth week of gestation and continuing until birth.
- Gametes are specialized reproductive cells involved in sexual reproduction, with male gametes called sperm and female gametes called eggs or ova.
- A zygote is the result of the fusion of two gametes during fertilization, representing the beginning of a new individual's development.