1. F. J. Browne (1942 ): Antenatal' and Postnatal Care. Churchill, London, p.160 .
2. J. B. Lee, and J. P. Greenhill (1947 ): Principles and Practice of Obstetrics. Saunders, Philadelphia, p.521 .
3. Dieckmann, and Davis. Stander (1945 ):Quoted byP. 143 .