The behavior is a normal developmental phase for children.
It might be possible to prove the origin of all classes from Pelmatozoa, without thereby explaining the origin of such fundamental features as radial symmetry, the developmental metamorphosis, and the torsion that affects both gut and body-cavities during that process; but the acceptance of a Dipleurula as the common ancestor necessitates an explanation of these features.
They thus provide a useful adjunct to cDNA libraries in giving access to a large number of genes independent of developmental stage.
What is the relationship between developmental verbal dyspraxia and childhood apraxia of speech?
The " jewel in the crown " of the developmental work was the involvement of lay assessors.