The lineage was traced by direct observation. Because the animal is transparent, researchers watched divisions under a microscope and reconstructed the complete cell genealogy from zygote to adult. The same divisions occur in the same order in every individual, so the adult hermaphrodite reliably has 959 somatic nuclei and the male 1,031.
The count is smaller than the number of cells generated. During development 131 cells in the hermaphrodite die by programmed cell death, always the same cells. Identifying the genes controlling those deaths established apoptosis as a genetically programmed process, work recognised by the 2002 Nobel Prize in Physiology or Medicine shared by Sydney Brenner, Robert Horvitz and John Sulston.
The nervous system was mapped in the same spirit. The hermaphrodite has 302 neurons, and their synaptic connections were reconstructed from serial electron micrographs to produce the first complete connectome of any animal. Having the wiring diagram has not, however, delivered a full account of behaviour, because synaptic structure alone does not specify signal strength, neuromodulation or state.