The embryos are usually observed on the 2nd-3rd day after egg retrieval. The following criteria are used for scoring the embryos (Class I and II embryos are considered to be of high quality): Class I: uniform size of the cleavage bulb, regular shape, intact zona pellucida; uniform and clear cytoplasm, no granulation; 0-5% fragmentation. Grade II: slightly uneven size and irregular shape of the ovoid spheres; cytoplasm with granulation; fragmentation in the range of 6%-20%. Grade III: ovoid spheres were obviously uneven in size and irregular in shape; cytoplasm had coarse granules; fragmentation was 21%-50%. Grade IV: the size of the ovoid sphere is seriously uneven; the cytoplasm may have severe granularity; the fragmentation is more than 50%. The figure below shows, from left to right, the grade I, II, III and IV embryos on day 2 after egg retrieval.