Complex sentence: All the regional divisions of WHO were created between 1949 and 1952, and are based on article 44 of WHO's constitution, which allowed the WHA to "establish a (single) regional organization to meet the special needs of (each defined) area".
Split into simpler sentences: All the regional divisions of WHO were created between 1949 and 1952. They are based on article 44 of WHO's constitution which allows the WHA to "establish a (single) regional organization to meet the special needs of (each defined) area".