Cell interactions coordinate the development of the C. elegans egg-laying system. 
Egg laying by the nematode Caenorhabditis elegans requires the functioning of the vulva, the gonad, the egg-laying muscles, and the two HSN neurons, which innervate these muscles.  By analyzing a newly isolated mutant (dig-1) that displaces the gonad, we discovered that cell interactions coordinate the spatial relationships among the different components of the egg-laying system.  First, the gonad induces the formation of the vulva, and vulval induction by dorsal gonads strongly suggests that the inductive signal can act at a distance.  Second, the gonad acts at a distance to regulate the migrations of the sex myoblasts that generate the egg-laying musculature.  Third, the positions of the axonal branch and synapses of each HSN neuron are displaced correspondingly with the rest of the egg-laying system in dig-1 animals, which suggests that cell interactions also control aspects of HSN development.