Those neurons did not make the correct connections must die. This phenomenon points out one very important conclusion. The human brain has a network structure which preexists the neurons stampede. Those migrating neurons must fit into that pre-designed structure or face the death. This pre-designed brain network must be topological in nature, so does the laws of body shape.
Most plants do have four direction symmetry because they are unable to move around.
Being able to turn round and round quickly, animals give up the front back symmetry because of the biological economy. But the symmetry breaking has much deeper metaphysical reason than just the simple biological economy. The outer cell divides again slightly obliquely to give an external cell that makes the socket, and an internal cell that makes the scale.