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Fertilization is the joining of sperm and egg to produce a diploid zygote. You might think of fertilization as the event that begins development. But keep two things in mind: (1) in asexually reproducing animals, development proceeds without fertilization, and (2) in animals where fertilization does occur, it is preceded by critical events in the maturing egg that influence subsequent development. Thus in studying fertilization we are asking how it activates multicellular development in sexually reproducing animals.
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The egg and sperm contribute different materials to the zygote.
Sperm entry converts the symmetry of the frog egg from radial to bilateral.
The movement of cytoplasm, proteins, and organelles following fertilization changes the distribution of critical signaling factors.
Fertilization does more than restore a full diploid complement of maternal and paternal genes. The fusion of sperm and egg cell membranes:
triggers ion fluxes across the egg membrane.
creates blocks to the entry of additional sperm into the egg.
changes the pH of the egg cytoplasm.
increases egg metabolism and stimulates protein synthesis.
initiates the rapid series of cell divisions that produce a multicellular embryo.
We described the mechanisms of fertilization in Key Concept 42.2. Here we will take a closer look at the cellular and molecular interactions of sperm and egg that initiate the first steps of development.