During development, reductions in the numbers of axons, cells, neurons, and synapses (quantitative regression), and the loss of structures and their replacement by new ones, with possibly new functions (qualitative regression). Behavioral examples of both are less clear-cut and in particular with regard to human development. A major problem, especially for the different forms of qualitative regression, is to know what happens to those structures that have been replaced by others.
See Apoptosis, Axon retraction (or pruning), Cajal-Retzius cells, Egg-tooth (or carbuncle), Metamorphosis (or indirect development), Phase transition (or shift), Polyneural to mononeural innervation, Progress, Quantitative and quantitative change, Radial glial cells, Regressive event, U-shaped learning, Stepping response