1. 1. M. Riley, and J. Halliday, Birth Defects in Victoria 1983-1998. Perinatal Data Collection Unit , Victorian Government Department of Human Services: Melbourne, 2000 :51 -59 .
2. 2. RK. Hall, Paediatric orofacial pathology and medicine . London: Chapman and Hall, 1994 :87 -99 .
3. 3. KL. Moore, The developing human; clinically orientated embryology . 4th edn. Philadelphia: WB Saunders, 1988 :195 -203 .