in which reproductive pattern the zygote is surrounded by some secreations along with some food reserve and a shell
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Fertilization—Transfer and Fusion of Gametes
Fertilization occurs when a spermatozoon and an ovum unite to form a diploid zygote. External fertilization occurs when this union occurs outside the bodies of the male and female, and internal fertilization occurs when the union occurs within the female’s body, almost always in the oviducts.
Males produce millions of tiny spermatozoa, whereas females produce relatively few eggs. Even though eggs of some amphibians are small, they are orders of magnitude larger than spermatozoa. During mating, many sperm reach the surface of an egg but only one penetrates the cell membrane of the ovum to fertilize it. When sperm first arrive at the egg, a few adhere to the surface. Enzymes produced by the acrosome digest a tiny hole in the egg capsule, bringing the sperm head into contact with the plasma membrane. The enzymes break down receptors binding the sperm pronucleus to the surface of the egg, and the sperm pronucleus moves into the cytoplasm of the ovum. In response to the entry of the sperm pronucleus, the vitelline membrane separates and elevates, lifting all other sperm from the ovum’s surface. As the successful sperm pronucleus moves to the ovum pronucleus, the ovum pronucleus completes its final meiotic division. The fusion of the two pronuclei is the final stage of fertilization and restores the diploid (2N) condition to the fertilized ovum, which is thereafter called the zygote. The zygote soon begins development via typical cell division—mitosis. Embryonic development continues in externally fertilized eggs (amphibians), but developmental arrest occurs in internally fertilized eggs (reptiles) after development to a gastrula stage. Salamanders are unusual because they have polyspermic fertilization, in which more than one sperm pronucleus enters the ovum’s cytoplasm, but only one sperm pronucleus fuses with the egg pronucleus.