At postnatal day two (P2), the immature calyx of Held is formed, easily distinguished by its characteristic sealed-spoon morphology. The primary synaptic contacts that form the calyx are assembled between neurons of the MNTB (medial nucleus of the trapezoid body) and VCN (ventral cochlear nerve), eventually connecting with one another by projecting across the midline of the two areas. These associations begin to appear immediately after VCN neurons have been generated; one can observe the earliest formation of these contacts around embryonic day 17 (E17). These neuronal connections, which make up an important area of the cochlea, form branches with one another that terminate in the calyx of Held. Over the course of the next two to three weeks, the neuronal contacts that first formed the embryonic calyx evolve in shape and function, culminating in a mature calyx that facilitates the consistent, rapid spread of signals in the MNTB-VCN area.