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Morphological similarity in vertebrate embryos
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.
The hourglass model
The observation of the conservation of animal morphology during the embryonic phylotypic period, where there is maximal similarity between the species within each animal phylum, has led to the proposition that embryogenesis diverges more extensively in the early and late stages than the middle stage, and is known as the hourglass model. Comparative genomic studies suggest that the phylotypic stage is the maximally conserved stage during embryogenesis.    
- 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". Zebrafish Information Network. Retrieved 2007-07-18.
- Haeckel E (1874) Anthropogenie oder Entwickelungsgeschichte des Menschen. Engelmann, Leipzig
- Duboule D. 1994. Temporal colinearity and the phylotypic progression: a basis for the stability of a vertebrate Bauplan and the evolution of morphologies through heterochrony. Dev Suppl:135–142.
- Prud'homme B, Gompel N. (2010). "Evolutionary biology: Genomic hourglass". Nature 468 (7325): 768–9. doi:10.1038/468768a. PMID 21150985.
- Irie N, Sehara-Fujisawa A. (2007). "The vertebrate phylotypic stage and an early bilaterian-related stage in mouse embryogenesis defined by genomic information.". BMC Biology 5:1. doi:10.1186/1741-7007-5-1. PMID 17222327.
- Domazet-Lošo T, Tautz D. (2010). "A phylogenetically based transcriptome age index mirrors ontogenetic divergence patterns". Nature 468 (7325): 815–8. doi:10.1038/nature09632. PMID 21150997.
- Irie N, Kuratani S. (2011). "Comparative transcriptome analysis reveals vertebrate phylotypic period during organogenesis.". Nature Communications 2 (248). doi:10.1038/ncomms1248. PMID 21427719.