Editing Embryology (section)

== Comparative embryology ==

=== 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]]).

=== Basal phyla ===
Animals that belong to the basal phyla have holoblastic radial cleavage which results in radial symmetry (see: [[Symmetry in biology]]). During cleavage, there is a central axis that all divisions rotate about. The basal phyla also has only one to two embryonic cell layers, compared to the three in bilateral animals ([[endoderm]], [[mesoderm]], and [[ectoderm]]).

=== Bilaterians ===
In [[Bilateria|bilateral animals]], cleavage can be either holoblastic or meroblastic depending on the species. During [[gastrulation]], the [[blastula]] develops in one of two ways that divide the whole animal kingdom into two-halves (see: ''[[Embryological origins of the mouth and anus]]''). If in the [[blastula]], the first pore, or [[blastopore]], becomes the mouth of the animal, it is a [[protostome]]; if the blastopore becomes the anus, then it is a [[deuterostome]]. The [[protostomes]] include most [[invertebrate]] animals, such as insects, worms and molluscs, while the [[deuterostome]]s include a few invertebrates such as the [[echinoderms]] (starfish and relatives) and all the [[vertebrates]]. In due course, the [[blastula]] changes into a more differentiated structure called the [[gastrula]]. Soon after the [[gastrula]] is formed, three distinct layers of cells (the [[germ layers]]) from which all the bodily organs and tissues then develop.

==== Germ layers ====
* The innermost layer, or [[endoderm]], give rise to the digestive organs, the gills, lungs or swim bladder if present, and kidneys or nephrites.
* The middle layer, or [[mesoderm]], gives rise to the muscles, skeleton if any, and blood system.
* The outer layer of cells, or [[ectoderm]], gives rise to the nervous system, including the brain, and skin or carapace and hair, bristles, or scales.

====''Drosophila melanogaster'' (fruit fly)====
Drosophila have been used as a developmental model for many years. The studies that have been conducted have discovered many useful aspects of development that not only apply to fruit flies but other species as well.

Outlined below is the process that leads to cell and tissue differentiation.

# [[Maternal effect#Maternal effects in genetics|Maternal-effect genes]] help to define the anterior-posterior axis using [[Bicoid (gene)]] and [[Nanos (gene)]].
# [[Gap gene]]s establish 3 broad segments of the embryo.
# [[Pair-rule gene]]s define 7 segments of the embryo within the confines of the second broad segment that was defined by the gap genes.
# [[Segment-polarity gene]]s define another 7 segments by dividing each of the pre-existing 7 segments into anterior and posterior halves using a gradient of [[Hedgehog signaling pathway|Hedgehog]] and [[Wnt signaling pathway|Wnt]].
# [[Hox gene|Homeotic (Hox) genes]] use the 14 segments as pinpoints for specific types of cell differentiation and the histological developments that correspond to each cell type.

====Humans====
{{Main|Human embryonic development}}
Humans are [[Bilateria|bilateral animals]] that have holoblastic rotational cleavage. Humans are also [[deuterostome]]s. In regard to humans, the term embryo refers to the ball of dividing cells from the moment the [[zygote]] implants itself in the [[uterus]] wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.