Embryos are generally obtained through blastomere or blastocyst biopsy. Fertilized eggs are sufficiently large to perform microinjections and blastomere isolations with relative ease. In twinning assays, molecules are injected into the ventral blastomere of a four-cell stage embryo. These precursors typically appear when the blastomere differentiates into the 8- and 16-cell masses. Blastomere biopsy is a technique in which blastomeres are removed from the zona pellucida.