Professional services through officers, employees, and

agents. A professional corporation shall render professional services only through its officers, employees and agents who are duly licensed or otherwise legally authorized to render such professional services; provided, however, this section shall not be interpreted to include in the term "employee", clerks, secretaries, bookkeepers, technicians and other assistants who are not usually and ordinarily considered by custom and practice to be rendering professional services to the public for which the license is required.