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Ontogenetic Depth

Ontogenetic depth is a measure of the distance (in terms of cell division and differentiation) between a single-celled state and an adult animal (metazoan) capable of reproduction. All animals begin their existence as a single cell, the fertilized egg. From that cell, many other cells arise, and become specialized for particular functional roles -- for instance, as sensory organs, skeletal parts, or reproductive structures (such as ovaries). The ontogenetic depth of any species measures the developmental distance between the starting point, the egg, and the stage at which organisms in the species can successfully reproduce themselves.

Developmental biology has mapped the ontogenetic depth of a handful of species, in the so-called "model systems" of the discipline, such as the nematode Caenorhabditis elegans or the fruit fly Drosophila melanogaster. To explain the historical origin of any animal species (and arguably the same is true for plants), one must give an account of how that species' respective ontogenetic network -- i.e., its process of differentiation -- was constructed.

Editor(s): Paul Nelson

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