The migratory phenomenon is a process unchaining unprecedented forces in the formation of the Mexican urban structure. To master these forces and convert them into a tool that will properly shape the nation, it is necessary to assess a directing concept. This thesis is a search for a feasible concept. Migration is in depth a blind search for urban conditions. Today, these conditions must be proliferated to an enlarging number of settlements. The quality of the city must be given to the town. The search is conducted through three processes: I. A time-past -Tributary process or analytical approach II. A time-less Directing process or theoretical approach III. A time-future Formative process or design approach. The forces at work are the time present; the relationship between the past with the future. As a response from these processes, a basic proposition is made: The gradual change toward an urban condition can be achieved by function- ally linking series of existing towns. This linkage pattern is here referred to as SIorganic solidarity". In this concept of organic solidarity, each town requires the other in order to perform its major function's. The resultant urban unit is as the concept of a molecule in physics: the smallest structural unit made of several different parts necessary to each other to remain balanced and at the same time coexisting as an integral part of a major whole; this whole is equilibrized by the balance of every one of its molecules. The existence of this urban uriit is transitory. The concept is a process of urban formation, not a form. The purpose of the demonstration design is to represent the concept in an urban form as a synthesis of proposals and ideas. The resultant image is only one interpretation. There can be others.