A zygote (from Greek ζυγωτός zygōtos "joined" or "yoked", from ζυγοῦν zygoun "to join" or "to yoke"), is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo. In single-celled organisms, the zygote divides to produce offspring, usually through Mitosis. Mitosis is the process of cell division.
A zygote is always synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development. Zygotes are usually produced by a fertilization event between two haploid cells—an ovum (female gamete) and a sperm cell (male gamete)—which combine to form the single diploid cell. Such zygotes contain DNA derived from both parents, and this provides all the genetic information necessary to form a new individual.
In land plants, the zygote is formed within a chamber called the archegonium. In seedless plants, the archegonium is usually flask-shaped, with a long hollow neck through which the sperm cell enters. As the zygote divides and grows, it does so inside the archegonium. zygote is formed due to fusion of sperm and ovum in oviduct.zygote is morula a mass of cells , then it undergo mitosis and cell divides and form blastocyst. zygote undergo divisions for four days and on fifth day it forms a blastocyst.
In mammalian reproduction, after fertilization has taken place the zygote travels down the fallopian tube, while dividing to form more cells without the zygote actually increasing in size. This cell division is mitotic, and is known as cleavage. All mammals go through the zygote stage of life. Mammalian zygotes eventually develop into a blastocyst, after which they are more generally termed an embryo, and then a fetus.
In other species 
See also 
- "English etymology of zygote". myetymology.com.
- Neas, John F. "Human Development". Embryology Atlas
- O’Reilly, Deirdre. "Fetal development". MedlinePlus Medical Encyclopedia (2007-10-19). Retrieved 2009-02-15.
- Klossner, N. Jayne and Hatfield, Nancy. Introductory Maternity & Pediatric Nursing, p. 107 (Lippincott Williams & Wilkins, 2006).
- Blastomere Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2012. Web. 06 Feb. 2012.
- Blackburn, Susan. Maternal, Fetal, & Neonatal Physiology, p. 80 (Elsevier Health Sciences 2007).
Oocyte + Sperm
|Stages of human development