By December 1, 1951, a tentative system of county primary roads shall be selected by the board of county road commissioners in each county and certified to the state transportation department for its approval. Such tentative system of county primary roads shall be selected on the basis of greatest general importance to the county and shall include any such county roads then legally established and existing as such within the limits of incorporated cities and villages. Each such tentative system of county primary roads certified to the state transportation department shall be checked and reviewed under its direction. Within 6 months after receipt by the department of each such certification, the state transportation department shall approve such part of that tentative system of county primary roads as the department determines is appropriate and shall certify to that board of county road commissioners the approved portion of the tentative system and any deletions therefrom. So much of the tentative system of county primary roads of any county as is approved by the state transportation department shall constitute the county primary road system of that county for all purposes and shall be officially known as the county primary road system of that county.