The starting point of all many-celled organisms is the one-celled ovum, or egg. Throughout the animal world there are hundreds of different types of eggs. Some have shells; others do not. There are large eggs and tiny ones. The number of eggs produced varies enormously from species to species. A codfish, for example, may lay up to 6 million eggs in one breeding season. A whale, by contrast, produces only one egg every two years.
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