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After the egg drops from the ovary, it travels through the Fallopian tube, where there's about a 24-hour window when it can be fertilized. Even if you try to summit Everest, this is the most dangerous journey you will ever take. If all goes right, the sperm meets the egg in the Fallopian tube and the two half-genomes unite to form a complete set of genes containing all of the DNA necessary to make a new human being. The fertilized egg says thank you very much and moves along to the uterus. There, it will attach to the uterine lining and begin the amazing process of becoming a baby. Once the sperm fertilizes the egg, the fragile blastocyst multiplies its cells and must implant on the uterus wall to endure the 280-day pregnancy.
Once the sperm penetrates the ovum (egg) a physiologic barrier, the zona pellucida is formed around the ovum so no additional sperm can penetrate. The moment of fertilization occurs as the nuclei of the ovum and the sperm swell, approach and ultimately unite. At this moment the nuclear membrane of the haploid gametes disappears, the chromosomes find their mate, the diploid chromosome number is restored and the zygote is created. At this moment the biological gender of the zygote is determined.
When a egg is fertilized with a sperm, an embryo is created. In this video, Darcy Bryan, MD, of Riverside Community Hospital, explains the fertilization process.
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