In normal fertilization, many sperm penetrate the corona radiata of the ovum (a layer of follicle cells surrounding the ovum). Then, typically only one sperm will penetrate the zona pellucida (a film of glycoproteins surrounding the oocyte) and reach the oocyte. The sperm’s membrane then fuses with the actual membrane of the oocyte. This fusion triggers changes in the oocyte (or rather, what was the oocyte) so that (a) the membrane of this new cell undergoes a rapid polarization, and (b) a calcium wave is produced throughout the new cell’s cytoplasm so that the zona pellucida hardens over approximately 30 minutes and repels penetration by sperm. These facts indicate that what is living at this point is not an ovum.
With the fusion of the sperm and the ovum, the tail of the sperm is lost, and the membrane surrounding the head of what was the sperm joins the surface membrane of the former oocyte, creating a single, continuous membrane. This allows cytoplasmic factors derived from the ovum to affect the nuclear contents derived from the sperm — for example, new types of histones begin to be associated with those chromosomes, modifying the behavior and interaction of the molecules in these chromosomes. This shows that the sperm has ceased to be.
At this point, the genetic material from the ovum (the female pronucleus) and the genetic material from the sperm (the male pronucleus) are both contained within a single new cell, are being moved toward each other, and will eventually intermingle. This is the point just after the fusion of the membranes of the sperm and the ovum, when the ovum and the sperm cease to be, and a new organism — a whole human organism — comes to be. National Review
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