1. E. Albrecht, and R. A. Willis (1904 ), Quoted by, "The Borderland of Embryology and Pathology ", 2nd edition , Butterworths, London: 351 .
2. R. A. Willis, R. Cameron, and G. Payling Wright (1962 ), "Pathology of Tumours of Children ", Oliver and Boyd, EdinburghandLondon: 93 , 93 .