Editing Development of the human body (section)

===Fertilization===
{{Main|Human fertilization}}
[[Image:Sperm-egg.jpg|right|thumb|Sperm fertilizing an egg]] 
Fertilization occurs when the sperm successfully enters the ovum's membrane. The [[chromosome]]s of the sperm are passed into the egg to form a unique genome. The egg becomes a [[zygote]] and the germinal stage of embryonic development begins. The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation. The germinal stage is over at about 10 days of gestation.<ref name="germinal stage">{{cite web|title=Germinal stage|url=http://medical-dictionary.thefreedictionary.com/germinal+stage|work=Mosby's Medical Dictionary, 8th edition.|publisher=Elsevier.|access-date=6 October 2013|archive-date=28 April 2021|archive-url=https://web.archive.org/web/20210428005411/http://medical-dictionary.thefreedictionary.com/germinal+stage|url-status=live}}</ref>

The [[zygote]] contains a full complement of [[genetic material]] with all the biological characteristics of a single human being, and develops into the embryo. Embryonic development has four stages: the [[morula]] stage, the [[blastula]] stage, the [[gastrula]] stage, and the [[neurula]] stage. Prior to implantation, the [[embryo]] remains in a protein shell, the [[zona pellucida]], and undergoes a series of rapid mitotic cell divisions called [[cleavage (embryo)|cleavage]].<ref name="TortoraG">{{cite book |last1=Tortora |first1=G |last2=Derrickson |first2=B |title=Principles of anatomy & physiology |year=2012 |url=https://archive.org/details/principlesanatom00tort_737 |url-access=limited |publisher=Wiley |isbn=978-0470-91777-0 |page=[https://archive.org/details/principlesanatom00tort_737/page/n1218 1183]|edition=13th.}}</ref> A week after fertilization the embryo still has not grown in size, but hatches from the zona pellucida and adheres to the lining of the mother's [[uterus]]. This induces a [[decidual reaction]], wherein the uterine cells proliferate and surround the embryo thus causing it to become embedded within the uterine tissue. The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the [[fetal membranes]] and the [[placenta]].  In humans, the embryo is referred to as a [[fetus]] in the later stages of prenatal development.  The transition from embryo to fetus is arbitrarily defined as occurring 8 weeks after fertilization. In comparison to the embryo, the fetus has more recognizable external features and a set of progressively developing internal organs. A nearly identical process occurs in other species.