In developmental systems without feedback, the growth is polynomial and a hierarchy exists between subsystems according to the location of the generating subword along the generating word; as a consequence, the possibility of regeneration after partial destruction is limited and relative stability of the system structure is observed if one letter is changed in the generating word. In developmental systems whose generating word has feedback the growth is exponential, regeneration may be complete following the survival of a unique element and changing one letter in the generating word may cause a complete change in the structure of the system.