Sometimes referred to as "late cleavage," the blastula is a hollow ball of cells. The cells at this stage continue to undergo rapid cell division (mitosis-1) creating a hollow space within the blastula called the blastocoel (2). This transient structure which will soon disappear during gastulation performs the short but important task of preventing cell-cell communication between the ectoderm at the animal pole and the endoderm below in the vegetal pole. The point of sperm entrance will determine the anterior (A) and posterior (P) axis in the embryo. We can estimate the quadrant of sperm entrance based on the cortical layer (3). The counterclockwise rotation seen by the lower left margin of cortical layer (4) indicates that sperm entrance occured in the left quadrant of the animal pole (the cortical layer rotates toward the point of sperm entrance). Invagination will begin opposite of sperm entrance (5).