In the laboratory, the identified eggs are stripped of surrounding cells and prepared for fertilization. In the meantime, semen is provided by the male partner and prepared for fertilization removing inactive cells and seminal fluid. The sperm and the egg are incubated together (at a ratio of about 75,000:1) in the culture media for about 18 hours. By that time fertilization should have taken place and the fertilized egg would show two pronuclei. In situations where the sperm count is low a single sperm is injected directly into the egg using intracytoplasmic sperm injection (ICSI) (see below). The fertilized egg is passed to a special growth medium and left for about 48 hours until the egg has reached the 6-8 cell stage.Laboratory have developed grading methods to judge oocyte and embryo quality. Typically, embryos at the 6-8 cells stage are transferred three days after retrieval.