"An embryo is a new organism within the early stage of development. In sexually reproducing organisms for example in humans, the embryo is the resultant single cell post fertilisation of a female egg cell by the male sperm cell. An organism in its initial stage of development ready to grow and differentiate, in humans from approximately the first week post fertilisation to the end of eighth week. The unborn baby beyond eight weeks is referred to as a fetus.

Detailed Information
Immediately when fertilization takes place, the embryo develops from a zygote the single cell resulting from the fertilization of the female egg cell by the male sperm cell. The zygote possesses half the DNA of each of its two parents. Cellular division begins and progresses at a fast rate. At just about four weeks the cell mass becomes a recognizable embryo, which measures around 7 to 10 millimetre long with rudimentary organs, normally at this stage of the embryo extremities such as the eyes and ears are seen. At the end of around 8-9 weeks the embryo should be a fully grown to approximately 2.5 cm, with head being a prominent part as a result of the fast development of the brain; the sex is distinguished at this stage too.

Embryo Development
The development of a zygote cell into an embryo happens through specific stages namely blastula, gastrula, and organogenesis. Blastula stage usually displays a fluid-filled cavity, the blastocoel, engulfed by a sheet of cells called as blastomeres. In humans the ovum/eggs are fertilised in the fallopian tube through which it reaches the uterus. An embryo is then referred to as fetus at a more advanced stage of development up until the birth.

Cells from blastula stage undergo a complex process of cell division, invasion and/or migration to form two or three tissue layers scientifically referred to as diploblastic and triploblastic tissue layers respectively. The three tissue layers are called endoderm, ectoderm and mesoderm. The position and arrangement of the layers are species specific. Humans being vertebrates, a specific population of embryonic cells called neural crest has been proposed as a fourth tissue layer, helping in the growth of head structures.

Organogenesis stage marks the interactions between tissue layers, combined with the ability to differentiate further in to organ-specific cell varieties, for instance, sub population of these tissue layers develop into specific organs like brain, nerves, spinal cord and etc.

Research and Technology
Embryos utilized in numerous fields of research and in techniques of assisted reproductive technology. An egg can be fertilised in vitro, and the resulting embryo could frozen for later use. Stem cell research, cloning, DNA engineering, prenatal diagnostics and testing are current potential areas of research.
Cryopreservation is currently an accepted technology for preservation of animal embryos and genetic material. Embryos are collected, processed and stored at extreme low temperatures with the intent of later use and research.
Ethical and legal communities to date continue to debate the research and use of human embryos."