Why ontogeny recapitulates phylogeny.
In development, mammalian embryos successively develop notochord, somites, gill pouches and three sets of kidneys: pronephros, mesonephros and metanephros. Hence, Ernst Haeckel's dictum "ontogeny recapitulates phylogeny." The reason for this dictum appears to be the immortality of genes. Genes, once well entrenched, appear to linger for a long time even after the forfeiture of their raison d'être. The genes for dental enamel and dentin of the chicken are good examples. Furthermore, genes once doomed may resurrect themselves. For example, genes for red and green opsins of higher primates arose as alleles of the once doomed but resurrected red opsin locus.
Ohno
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Beckman Research Institute of the City of Hope, Duarte, CA 91010-3000, USA.
D000595:Amino Acid Sequence; D000818:Animals; D002642:Chick Embryo; D005314:Embryonic and Fetal Development / Q000235:genetics; D016678:Genome; D006801:Humans; D008969:Molecular Sequence Data; D010802:Phylogeny; D011003:Ploidies; D017386:Sequence Homology, Amino Acid; D014714:Vertebrates / Q000196:embryology / Q000235:genetics*