A system for sorting X and Y sperm cells, the system being characterised in that it includes: a variable rate fluid delivery system for delivering a fluid stream containing X and Y sperm cells; a flow cytometry apparatus for: receiving the stream and forming droplets containing the sperm cells, the droplets including first droplets each containing one or more X sperm cells, second droplets each containing one or more Y sperm cells, and third plurality of droplets each containing one or more X sperms cells and one or more Y sperm cells; sorting the first droplets from the second and third droplets; collecting the first droplets to provide at least one population of X sperm cells; and identifying a quantity of X sperm cells in the at least one population; and a control responsive to instructions received from the flow cytometer apparatus for varying the rate at which the fluid is delivered to the flow cytometer apparatus as a function of the quantity of identified X sperm cells in the said at least one population relative to the total number of X cells in the first, second and third droplets, and wherein the control is operable to increase the rate of fluid delivery when the quantity of X cells in the said at least one population is above the acceptable quantity and to decrease the rate of fluid delivery when the quantity of X cells in the said at least one population is below the acceptable quantity. a variable rate fluid delivery system for delivering a fluid stream containing X and Y sperm cells; a flow cytometry apparatus for: receiving the stream and forming droplets containing the sperm cells, the droplets including first droplets each containing one or more X sperm cells, second droplets each containing one or more Y sperm cells, and third plurality of droplets each containing one or more X sperms cells and one or more Y sperm cells; sorting the first droplets from the second and third droplets; collecting the first droplets to provide at least one population of X sperm cells; and identifying a quantity of X sperm cells in the at least one population; and receiving the stream and forming droplets containing the sperm cells, the droplets including first droplets each containing one or more X sperm cells, second droplets each containing one or more Y sperm cells, and third plurality of droplets each containing one or more X sperms cells and one or more Y sperm cells; sorting the first droplets from the second and third droplets; collecting the first droplets to provide at least one population of X sperm cells; and identifying a quantity of X sperm cells in the at least one population; and a control responsive to instructions received from the flow cytometer apparatus for varying the rate at which the fluid is delivered to the flow cytometer apparatus as a function of the quantity of identified X sperm cells in the said at least one population relative to the total number of X cells in the first, second and third droplets, and wherein the control is operable to increase the rate of fluid delivery when the quantity of X cells in the said at least one population is above the acceptable quantity and to decrease the rate of fluid delivery when the quantity of X cells in the said at least one population is below the acceptable quantity.