MODEL RELEASED. In vitro fertilization. Technician using a microscope to observe defrosting embryos being prepared for uterus implantation during in vitro fertilization (IVF) treatment. The embryos have been stored in liquid nitrogen at -196 degrees Celsius and must be rapidly defrosted in order to maintain their integrity. They are at an early developmental stage known as a blastocyst. They were created by the micro-injection of sperm cells into eggs removed from a woman's ovaries. The resulting fertilized eggs (zygotes) were allowed to develop into blastocysts in the laboratory, and will be implanted into the woman's uterus in order to conceive a child.