Editing Development of the human body (section)

===Embryonic development===
{{Main|Human embryonic development}}
[[Human embryonic development]] refers to the development and formation of the human [[embryo]]. It is characterised by the process of [[cell division]] and [[cellular differentiation]] of the [[embryo]] that occurs during the early stages of [[Human development (biology)|development]]. In biological terms, human development entails growth from a one-celled [[zygote]] to an adult [[human being]]. [[Fertilisation|Fertilization]] occurs when the [[sperm]] cell successfully enters and fuses with an egg cell ([[ovum]]).<ref name="germinal stage"/> The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of [[prenatal development]] commences. The embryonic stage covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a [[fetus]].

The germinal stage refers to the time from fertilization through the development of the early embryo until [[implantation (human embryo)|implantation]] is completed in the [[uterus]]. The germinal stage takes around 10 days.<ref name="germinal stage"/> During this stage, the zygote begins to divide, in a process called [[cleavage (embryo)|cleavage]]. A [[blastocyst]] is then formed and implanted in the [[uterus]]. Embryonic development continues with the next stage of [[gastrulation]], when the three [[germ layers]] of the embryo form in a process called [[histogenesis]], and the processes of [[neurulation]] and [[organogenesis]] follow.

In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. The entire process of embryonic development involves coordinated spatial and temporal changes in [[gene expression]], [[cell growth]] and [[cellular differentiation]]. A nearly identical process occurs in other species, especially among [[chordate]]s.