Section 51. There shall be a board of registration of sanitarians, hereafter in this section and in sections fifty-two and fifty-three referred to as the board, consisting of seven members to be appointed by the governor, of whom one shall be an employee of the department of public health, one shall be an educator in the field of sanitary science, four of whom shall be registered sanitarians, with at least five years experience in the field of environmental sanitation, and one of whom shall be a representative of the public, subject to the provisions of section nine B. Each member shall serve for a term of three years.