Pharyngula

In developmental biology, the pharyngula is a stage in embryonic development.cite book |author=Gilbert SF |title=Developmental mechanisms of evolutionary change. "in:" Developmental Biology |edition = 6th ed. |publisher=Sinauer Associates |year=2000 |isbn=0-87893-243-7 ] Named by William Ballard,cite journal | author = Ballard WW | title = Morphogenetic Movements and Fate Maps of Vertebrates | journal = American Zoologist | year = 1981 | volume = 21 | issue = 2 | pages = 391–9 | url = http://icb.oxfordjournals.org/cgi/content/abstract/21/2/391 | doi = 10.1093/icb/21.2.391] the pharyngula stage follows the blastula, gastrula and neurula stages. At the pharyngula stage, all vertebrate embryos show remarkable similarities,cite web
url = http://zfin.org/zf_info/zfbook/stages/phar.html
title = ZFIN Pharyngula Period Description
author = Sprague, J., "et al."
publication = Zebrafish Information Network
date = 2006
accessdate = 2007-07-18] containing the following features:
* notochord
* dorsal hollow nerve cord
* post-anal tail, and
* a series of paired branchial grooves.

The branchial grooves are matched on the inside by a series of paired gill pouches. In fish, the pouches and grooves eventually meet and form the gill slits, which allow water to pass from the pharynx over the gills and out the body.

In the other vertebrates, the grooves and pouches disappear. In humans, the chief trace of their existence is the eustachian tube and auditory canal which (interrupted only by the eardrum) connect the pharynx with the outside of the head.

Controversy

The existence of a common pharyngula stage for vertebrates was first proposed by German biologist Ernst Haeckel (1834–1919) in 1874. [Haeckel E (1874) Anthropogenie oder Entwickelungsgeschichtedes Menschen. Engelmann, Leipzig] However, more recent work has cast doubt on the validity of this proposed common stage. A detailed analysis of Haeckel's drawings and of variations in timing of appearance of various structures common to vertebrate embryos (analysis of sequential heterochrony) suggests that it may not be possible to find a single set of criteria that define a common pharyngula stage after all. [cite journal |author=Richardson MK, Hanken J, Gooneratne ML, "et al" |title=There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development |journal=Anat. Embryol. |volume=196 |issue=2 |pages=91–106 |year=1997 |pmid=9278154 |doi=]

ee also

*Evolutionary developmental biology
*Embryogenesis
*Embryo drawing
*Recapitulation theory
*Icons of Evolution

References

External links

Critiques of Richardson's conclusions can be found in this article [http://pharyngula.org/index/weblog/comments/wells_and_haeckels_embryos/ Wells and Haeckel's Embryos] and [http://sandwalk.blogspot.com/2007/03/happy-birthday-pz-myers.html here] . A discussion more supportive of Richardson's position can be found in this [http://8e.devbio.com/article.php?id=242 text book extract] .