Pharyngula

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This article is about the stage in embryonic development. For the science blog by PZ Myers, see Pharyngula (blog).

In developmental biology, the pharyngula is a stage in embryonic development.^[1] Named by William Ballard,^[2] the pharyngula stage follows the blastula, gastrula and neurula stages. At the pharyngula stage, all vertebrate embryos show remarkable similarities,^[3] 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.

1 Controversy
2 See also
3 References
4 External links

[edit] Controversy

The factual accuracy of this article is disputed.
Please see the relevant discussion on the talk page. (March 2008)

The existence of a common pharyngula stage for vertebrates was first proposed by German biologist Ernst Haeckel (1834–1919) in 1874.^[4] 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.^[5]

[edit] See also

[edit] References

^ Gilbert SF (2000). Developmental mechanisms of evolutionary change. in: Developmental Biology, 6th ed., Sinauer Associates. ISBN 0-87893-243-7.
^ Ballard WW (1981). "Morphogenetic Movements and Fate Maps of Vertebrates". American Zoologist 21 (2): 391–9. doi:10.1093/icb/21.2.391.
^ Sprague, J., et al. (2006). ZFIN Pharyngula Period Description. Retrieved on 2007-07-18.
^ Haeckel E (1874) Anthropogenie oder Entwickelungsgeschichte des Menschen. Engelmann, Leipzig
^ Richardson MK, Hanken J, Gooneratne ML, et al (1997). "There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development". Anat. Embryol. 196 (2): 91–106. PMID 9278154.

[edit] External links

Critiques of Richardson's conclusions can be found in this article Wells and Haeckel's Embryos and here. A discussion more supportive of Richardson's position can be found in this text book extract.