Editing Embryology (section)

=== 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.