It’s only when enough neurons have migrated into an optimally sized cluster that there is collectively enough of the stuff to prompt the neurons in the cluster to start forming dendrites, axons, and synapses with each other. —Once this local network is wired up (detectable by, say, a certain density of synapses), a chemorepellent is secreted, which now causes neurons to stop making connections to their neighbors, and to instead start sending long projections to other clusters,
