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		The membrane depolarization acts as the fast blockto polyspermy (fertilization by more than one sperm); it is though to interfere with electrostatic interactions between surface proteins of the sperm and egg (Fig. 7.22). Polyspermy is bad because, in addition to the extra set of chromosomes, a sea urchin sperm donates a centriole. The presence of additional centrioles during the first cell division will result in additional cleavage furrows and incorrect partitioning of the chromosomes (Fig. 7.21). Ca++ ions are released from the endoplasmic reticulum in a wave beginning a the sperm binding point (Fig. 7.25). The wave of Ca++ induces the fusion of a set of exocytic vesicles (known as the cortical granules) with the plasma membrane (Fig. 7.24). The contents of the cortical granules, released into the space between the vitelline membrane and the plasma membrane, include a trypsin-like protease that clips protein "posts" anchoring the vitelline membrane and removes the sperm-binding sites. The cortical granules also release mucopolysaccharides (carbohydrates) that absorb water and cause the vitelline membrane to expand and become the fertilization envelope; this is the slow blockto polyspermy (Fig. 7.23). The sperm and egg plasma membranes also fuseand the sperm nucleus enters and fuses with the egg nucleus. The wave of Ca++ and the rise in intracellular pH activate the metabolismof the fertilized egg (Fig. 7.27). The union of sperm and eggs creates single cell, thezygote, which gives rise to a new organism through the process of embryogenesis. The formation of a new organism requires both morphogenesis(the creation of form) and cell differentiation. In most animals, embryogenesis can be divided into three major stages: (1) Cleavage, in which rapid cell divisions divide the mass of the zygote into many cells and establish the basic embryonic axis; (2) Gastrulation, in which the gut is formed and the embryo becomes multilayered; and (3) Organogenesis, in which the major organ systems are formed. The embryo cannot feed until it develops a digestive system, so all of the nutrition that it will need must be packed in the egg. This is true in the sea urchin, but the food material (yolk) does not disrupt the partitioning of cytoplasm, known as cytokinesis, after each cell division. This pattern of cell division (complete and equal) is known as holoblastic cleavage(Fig. 8.5).