User:Starwarsfan6/Ontogeny

User:Starwarsfan6/Ontogeny

Introduction
The developmental history includes all the developmental events that occur during the existence of an organism, beginning with the changes in the egg at the time of fertilization and events from the time of birth or hatching and afterward (i.e., growth, remolding of body shape, development of secondary sexual, etc).

Ontogeny, embryology and developmental biology, genetics, developmental psychology,

Etymology
The word ontogeny comes from the Greek on meaning "a being, individual; being, existence," and from the suffix -geny from the Greek -geniea, meaning "genesis, origin, mode of production."

History
Ernst Haeckel, born in Germany on February 16th, 1834, was a zoologist, evolutionist and strong supporter of Darwinism. Haeckel suggested that ontogeny briefly and sometimes incompletely recapitulated or repeated phylogeny in his 1866 book Generelle Morphologie der Organismen ("General Morphology of Organisms). The scientific community wasn't very convinced or interested in his ideas, so he turned to producing more publications.  Ernst Haeckel and others imagined development as producing new structures after the earlier additions to the developing organism have been established. He proposed that individual development followed developmental stages of previous generations and then these new generations would add something new to this process.  Haeckel also proposed a causal parallelism between an animal's ontogeny and its phylogeny.  He proposed a biogenetic law that ontogeny recapitulates phylogeny, based on the idea that the successive and progressive origin of new species was based on the same laws as the successive and progressive origin of new embryonic structures. According to Haeckel, development produced novelties, and natural selection would eliminate species that had become outdated or obsolete. Though his view of development and evolution wasn't justifiable, future embryologists tweaked and collaborated with Haeckel's proposals and showed how new morphological structures can occur by the hereditary modification of embryonic development. Marine biologist Walter Garstang reversed Haeckel's relationship between ontogeny and phylogeny, stating that ontogeny creates phylogeny, not recapitulates it.

Fertilization
In humans, the process of fetal development starts after sperm fertilizes an egg and they fuse, which kickstarts embryonic development. The fusion of egg and sperm into a zygote changes the surrounding membrane to not allow any more sperm to penetrate the egg, so multiple fertilizations can be prevented. Fusion of a zygote also activates the egg so it can begin undergoing cell division.

Cleavage
Not long after successful fertilization by sperm, the zygote undergoes many mitotic divisions, which are also non-sexual cell division. Cleavage is the process of cell division, so the starting zygote becomes a collection of identical cells which is a morula and contains cells called blastomeres. Cleavage prepares the zygote to become an embryo, which is from 2 weeks to 8 weeks after conception (fertilization) in humans.

Blastulation
After the zygote has become an embryo, it continues dividing into a hollow sphere of cells, which is a blastula. These outer cells form a single epithelial layer, the blastoderm, that essentially encases the fluid-filled inside that is the blastocoel. The figure above shows the basic process that is modified in different species. Blastulation differs slightly in different species, but in mammals, the eight-cell stage embryo forms into a slightly different type of blastula, called a blastocyst. Other species such as sea stars, frogs, chicks, and mice have all the same structures in this stage, yet the orientation of these features differs, plus these species have additional types of cells in this stage.

Gastrulation
After blastulation, the single-layered blastula expands and reorganizes into multiple layers, a gastrula (seen in the figure above). Reptiles, birds and mammals are triploblastic organisms, meaning the gastrula comprises of three germ layers; the endoderm (inner layer), mesoderm (middle layer), and ectoderm (outer layer). As seen in the figure below, each germ layer will become multi-potent stem cells that can become a specific tissue depending on the germ layer and is what happens in humans. This differentiation of germ layers differs slightly, because not all of the organs and body parts below are in all organisms, but corresponding body systems can be substituted in place of these. In humans, the endoderm cells become the internal linings of organisms, such as the stomach, colon, small intestine, liver, and pancreas of the digestive system and the lungs. Next, the mesoderm gives rise to other tissues not formed by the ectoderm, such as the heart, muscles, bones, blood, dermis of the skin, bone marrow, and the urogenital system. This germ layer is more specific for species, as it is the distinguishing layer of the three that can identify evolutionarily higher life-forms (such as bilateral organisms like humans) from lower-life forms (with radial symmetry). Lastly, the ectoderm is the outer layer of cells that become the epidermis and hair while being the precursor to the mammary glands, central, and peripheral nervous systems.



Neurulation
In vertebrate organism embryos only, a neural tube is formed through either primary or secondary neurulation.

Organogenesis
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Metamorphosis and onward
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