Zur Geschichte der menschlichen Rückenmarkes und der Nervenwurzeln.

This is generally considered the first clear statement that nerve cells in the brain might be independent units rather than forming an anastomotic network—as Gerlach, Golgi, and most others believed—although there are earlier hints of this from a variety of sources.  His’s conclusion was based on embryological observations in the spinal cord, suggesting that axons arise from nerve cell bodies (one per nerve cell), followed later by dendrites, and that any fusion of processes, if they occur at all, must be a secondary event. (Larry W. Swanson).