Editing Embryology (section)

=== Preformationism and epigenesis ===
[[File:Preformation.GIF|thumb|upright|A tiny person (a ''homunculus'') inside a [[sperm]], as drawn by [[Nicolaas Hartsoeker]] in 1695]]As recently as the 18th century, the prevailing notion in western human embryology was [[preformation]]: the idea that semen contains an embryo – a preformed, miniature infant, or ''homunculus'' – that simply becomes larger during development.

The competing explanation of embryonic development was ''[[Epigenesis (biology)|epigenesis]]'', originally proposed 2,000 years earlier by [[Aristotle]]. Much early embryology came from the work of the [[Italy|Italian]] anatomists [[Ulisse Aldrovandi|Aldrovandi]], [[Aranzio]], [[Leonardo da Vinci]], [[Marcello Malpighi]], [[Gabriele Falloppio]], [[Girolamo Cardano]], [[Emilio Parisano]], [[Fortunio Liceti]], [[Stefano Lorenzini]], [[Spallanzani]], [[Enrico Sertoli]], and [[Mauro Ruscóni]]. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As [[microscopy]] improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.

==== Cleavage ====
[[Cleavage (embryo)|Cleavage]] is the very beginning steps of a developing embryo. Cleavage refers to the many mitotic divisions that occur after the [[Egg cell|egg]] is fertilized by the [[sperm]]. The ways in which the cells divide is specific to certain types of animals and may have many forms.

==== Holoblastic ====
[[Holoblastic|Holoblastic cleavage]] is the complete division of cells. Holoblastic cleavage can be radial (see: [[Radial cleavage]]), spiral (see: [[Spiral cleavage]]), bilateral (see: [[Bilateral cleavage]]), or rotational (see: [[Rotational cleavage]]). In holoblastic cleavage, the entire egg will divide and become the embryo, whereas in meroblastic cleavage, some cells will become the embryo and others will be the yolk sac.

==== Meroblastic ====
[[Meroblastic|Meroblastic cleavage]] is the incomplete division of cells. The division furrow does not protrude into the yolky region as those cells impede membrane formation and this causes the incomplete separation of cells. Meroblastic cleavage can be bilateral (see: [[Bilateral cleavage]]), discoidal (see: [[Discoidal cleavage]]), or centrolecithal (see: [[Centrolecithal]]).