This Vitrification Kit is based on technology developed by the Animal Reproduction and Biotechnology Laboratory at Colorado State University, Ft. Collins, Colorado. When using the enclosed material for long-term storage of vitrified equine embryos, it is essential to know the age and diameter of the embryos. It is highly recommended that embryos be 6 to 6.5 days of age (day 0 = day of ovulation), or approximately 8 days after administration of hCG. The ideal embryo stage for vitrification is a morula or early blastocyst. Ideally, the outer zona of the embryos should be less than 300 microns in diameter and, in the case of blastocysts, the zona should be no less than 1 /2 the normal thickness. It is highly recommended that an optical micrometer be used to measure the diameter of embryos to be vitrified. Embryos with an outer diameter of more than 300 microns will have a very low survival rate following vitrification.