Editing Pregnancy (mammals) (section)

===Cellular differentiation===
Following this, a narrow line of cells appears on the surface on the embryo. Its growth makes the embryo undergo [[gastrulation]], in which the three [[germ layer|primary tissue layers]] of the fetus, the [[ectoderm]], [[mesoderm]], and [[endoderm]], develop. The narrow line of cells begin to form the endoderm and mesoderm. The ectoderm begins to grow rapidly as a result of chemicals being produced by the mesoderm. These three layers give rise to all the various types of tissue in the body.

The endoderm later forms the lining of the [[tongue]], [[Gut (anatomy)|digestive tract]], [[lung]]s, [[Urinary bladder|bladder]] and several [[gland]]s. The mesoderm forms [[muscle]], [[bone]], and [[lymph]] tissue, as well as the interior of the lungs, [[heart]], and [[reproductive system|reproductive]] and [[excretory system]]s. It also gives rise to the [[spleen]], and produces blood cells. The ectoderm forms the [[skin]], [[nail (anatomy)|nail]]s, [[hair]], [[cornea]], lining of the internal and external [[ear]], [[nose]], [[Paranasal sinuses|sinus]]es, [[mouth]], [[anus]], [[tooth|teeth]], [[pituitary gland]], [[mammary gland]]s, [[eye]]s, and all parts of the [[nervous system]].

Approximately 18 days after fertilization, the embryo has divided to form much of the tissue it will need. It is shaped like a pear, where the head region is larger than the tail. The embryo's nervous system is one of the first organic systems to grow. It begins growing in a concave area known as the [[neural groove]].

The blood system continues to grow networks which allow the blood to flow around the embryo. Blood cells are already being produced and are flowing through these developing networks. Secondary blood vessels also begin to develop around the placenta, to supply it with more nutrients. Blood cells begin to form on the sac in the center of the embryo, as well as cells which begin to [[cellular differentiation|differentiate]] into blood vessels. [[endocardium|Endocardial]] cells begin to form the [[cardiac muscle|myocardium]].

At about 24 days past fertilization, there is a primitive S-shaped tubule heart which begins beating. The flow of fluids throughout the embryo begins at this stage.
<!-- add organ development section -->