I’ve never really looked into ‘ontogenetic depth’ before, so over lunch I dug out the original presentation and scanned through it.
This caught my eye:
Ontogenetic networks in all animals commence with a single cell, the fertilized egg. Then an unfolding arborescence of developmental decisions begins, whose complexity and overall architecture varies by taxon. In all animals, however, a point in the adult phenotype arrives when reproduction – the generation of gametes capable of fertilization – is possible. This distance, from the egg to the adult capable of reproduction, is what we term ontogenetic depth (see Figure 4). Somewhat more formally, ontogenetic depth may be defined as the distance, in terms of cell division and differentiation, between a unicellular condition and a macroscopic adult metazoan able to reproduce itself (i.e., generate gametes).
The ontogenetic depth of a handful of extant animals (from the model systems of developmental biology) is known with precision.
But what are those precisely known values for ontogenetic depth? The paper doesn’t say. Nor is any formula given. The closest thing to a precise value is this:
In the nematode Caenorhabditis elegans, for instance, a relatively small animal only 1.5 mm in length, 7 to 9 rounds of cell division lie between the fertilized egg and any cell in the adult: 959 somatic cells in the hermaphrodite (with a variable number of germ cells), and 1031 cells in the male (with its distinctive tale). For larger metazoans, of course, such as the dipteran Drosophila melanogaster, ontogenetic depth is much greater, as total cell number, degree
of cellular differentiation, and time to reproductive capability increase accordingly.
But what is the ‘ontogenetic depth’ of C elegans? Is it 7? 8±1? 959-1? 959/9? 1.5? Something else entirely?
Other questions spring readily to mind:
- Is D melanogaster one of the handful of organisms with a precisely known ‘ontogenetic depth’? If so, what is the precise value?
- What other organisms have known ‘ontogenetic depth’?
- How is ‘ontogenetic depth’ actually calculated? What units does it have?
- What is the male’s tale?
@pnelson later admitted that he didn’t even know how to measure ‘ontogenetic depth’, let alone calculate precise values:
Anyone would agree that there’s a distance between egg and adult in C. elegans. The question is how best to measure that distance, in units (or metrics) that allow comparison with other animal groups.
So the problem isn’t that ‘ontogenetic depth’ wasn’t ready, it’s that @pnelson claimed to have precise values for OD, then promised an explanation (which never materialized), but eventually admitted he didn’t know how OD could possibly be calculated.
The result is self-inflicted.