Two commissures develop in each neuromere, except the last (a9) neuromere which has only one commissure. Formation of the supraoesophageal and frontal commissures is delayed with respect to the commissures of the ventral cord, and they do not become evident until about 1 h after germ band shortening. Formally, all commissural growth cones have to perform a similar task, in fly as well as in man, as they grow towards and across the midline. Initially they have to show high affinity for cells located at the midline as they grow straight towards these cells. However, as soon as they have reached the midline they must loose their affinity for these cells, since they are now able to cross them and follow longitudinal axon tracts, which they had ignored on the ipsilateral side. This formal dissection of commissure formation implies general functions of midline cells during the attraction of commissural growth cones and their guidance across the midline.

After the specification of the midline cells the pioneers of the posterior commissure grow towards the anterior VUM neurons. Subsequently, the pair of MP1 neurons moves out of the midline to a more ventral and lateral position. The VUMs move slightly anteriorly and ventrally. When the growth cones of the posterior commissures reach the anterior VUM neurons they grow anteriorly around these cells. Once the posterior commissure is formed (note that the growth cones do not stop at the midline as it might look in the movie, they pass the midline and extend into the contralateral PNS or the connectives), the midline glial cells start to migrate posteriorly, coming into a position where they might serve as attractive intermediate target for the growth cones establishing the anterior commissure.The axons that pioneer the anterior commissure are initially found in close association with the posterior commissure. Only the growth cones of the VUM neurons separate the commissures. Subsequently, the posteriorly moving cell bodies of the MGA midline glia squeeze in between the two commissures, thereby forcing them apart. The intercalating migration of the two most posteriroly midline glial cells brings the two commissures into their final ladder like organisation. The initial 6 midline glia cells per metamer are reduced to 3-4 by apoptosis.