Progenesis

Progenesis is the speeding up of the germ line. The result is paedomorphosis - reproduction happens in what was ancestrally a juvenille morphologic stage.

Neoteny

Neoteny is the slowing down of somatic development. The result is that reproduction happens in what was ancestrally a juvenile morphologic stage. The age of breeding of the Mexican axolotl is not abnormally early and it is a reasonable inference that the time of reproduction has stayed constant, while somatic development has slowed down: the axolotl is probably neotenous.

Are humans paedomorphic?

Humans have also been argued to be neotenous. As adults, we are morphologically similar to the juvenile forms of great apes. This paedomorphosis, if it is real, would be neotenous rather than progenetic because our age of breeding has not shifted earlier relative to other apes; our age of first breeding is actually later than other apes. Our somatic development therefore has not simply slowed down while reproductive development has stayed the same: what happened was that our somatic development slowed down even more than our reproductive development.