AbstractIndividual ganglia of the cockroach embryo (Periplaneta americana) were explanted on clean glass coverslips immersed in a chemically defined liquid medium and incubated for periods up to eight weeks. Substantial, straight interganglionic connections were formed between: (1) rows of ganglia arranged in the normal in vivo configuration; (2) rows of ganglia placed in abnormal orders; (3) rows of ganglia which never form connections in vivo because they occur singly in the embryo; and (4) rows of ganglia in natural sequences but which have had their rostro‐caudal axes rotated 90° in relation to the line of the row. Therefore fascicles and interganglionic connectives were formed without regard to normal in vivo relationships.Daily observations with a Nomarski microscope indicated that several processes are involved in connective formation. (1) Initial outgrowth is in a random, radial pattern. (2) Intersecting fibers from adjacent ganglia are deflected toward each others' perikarya. (3) Initially bowed fiber connectives are straightened, perhaps by increases in fiber tension or by fiber shortening which may be brought about by neuronal or extraneuronal (glial) processes. (4) Outgrowing fibers follow already established fiber pathways.The present results indicate that fiber‐fiber and fiber‐target interactions play a significant role in the formation of interganglionic connectives. In this system, the spatial relationships between ganglia determine the patterns and varieties of permissible neuronal connections. Thus, major, straight nerve trunks may be formed between adjacent ganglia which are growing out fibers on a glass surface submerged in a liquid medium which offers minimal orientation cues and provides a growth substrate vastly different and simpler than that encountered by outgrowing fibers i