Semen separation. Technician using a centrifuge to separate the constituents of human semen. Semen consists of human sperm cells combined with fluid from the seminal vesicles and the prostate gland. The separation is done so sperm cells can be chosen for in vitro fertilisation (IVF). This is a technique used when a couple cannot conceive naturally. Healthy sperm cells are introduced to an egg cell in a laboratory. If fertilisation is successful, the resulting embryo is implanted directly into the mother's uterus.