Stage 6, Blastocyst.
A. Quality 1, Good. The cellular material occupies the entire perivitelline space. There are no major imperfections. There is a slight tendency for the embryo to be pyknotic, which may also be due to the amount of transmitted light. B. Quality 2, Fair. A degenerative clump of cells is present near the blastocele. The zona pellucida is misshapen. There is evidence of pyknosis. C. Quality 3, Poor. There are several areas of irregular and extruded cells. The embryo is asymmetrical. There is at least a 25% intact, viable embryonic mass. D. Quality 4, Degenerate. Dark ragged debris is present, as well as a lighter blastocele-like space at 6 o'clock.
Wright JM (2004)