The specific gravity (sp. gr.) of baboon spermatozoa was determined using the method described by Danon & Marikovsky (1964) for the determination of the density distribution of red blood cells and used by Lavon, Volcani, Amir & Danon (1966) for sp. gr. determination of bull spermatozoa and seminal plasma.Baboon semen was collected by electro-ejaculation using a rectal probe (Kraemer & Vera Cruz, 1969) and the ejaculate was fractionated within 5 min of ejaculation into a liquid and a coagulum fraction. Only the liquid fraction of the ejaculate was used for sp. gr. determinations. A battery of phthalate ester separating fluids of increasing sp. gr. (1·040 to 1·111) at increments of 0·002 to 0·003 was prepared. A column (10 mm) of separating fluid was drawn into capillary tubes
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