Flexible search for single-axon morphology.
(A) In stage 2, a neuron has several short neurites of similar length. (B) A developing neuron enters stage 3 if one neurite spontaneously elongates and develops to an axon. This established polarity is stably maintained. (C) Sometimes multiple neurites accidentally elongate, but this unfavorable pattern is flexibly reversed, leading to the single-axon pattern (B).