Definition: "The physical groove that forms between cells at the first cell division. One-celled oocytes (NF stage 1) undergo cytoplasmic reorganization driven by sperm entry in the animal pole, where the cleavage furrow initiates, and it moves circumferentially along a line of longitude as well as progressing inwards through the cytoplasm, ultimately dividing the fertilized oocyte into two cells of equal size, defining NF stage 2."