The organizational principles of secretory traffic have so far remained unclear, mostly because of the formidable complexity of the secretory pathway. In this study, I have combined the analysis of the simplest possible secretory event, a synchronized traffic wave crossing an individual Golgi stack, with the systematic use of electron tomography and morphometry to achieve a higher level of resolution of the traffic process than was previously possible. I have found that as a cargo wave enters the stack, it triggers the formation of tubules connecting successive cisternae which last for the duration of the wave and are pervious to Golgi enzymes but not to secretory cargo. In contrast, Golgi vesicles are depleted of both enzymes and cargo. Cargo then crosses the stack without leaving the lumen of the cisternae. These findings require a reassessment of the organization of intra-Golgi transport, and, more in general, introduce the principle of intercompartment continuities as important players in membrane traffic.