A cytometer system includes a fluid input manifold having a sample bore for accommodating a flow of sample fluid surrounded by a frustoconical shaped chamber for accommodating a flow of sheath fluid. A cuvette has a central bore positioned for receiving from the sample bore the flow of the sample fluid surrounded by and in non-mixing contact with the flow of the sheath fluid. The section of the central bore receiving the sample fluid and the sheath fluid has a frustoconical shape that converges and terminates in a section of the central bore that has a square cross section.