Much of this new knowledge has come from studying so-called model systems—organisms that biologists can easily mutate in the lab, such as the fruit fly Drosophila and the nematode Caenorhabditis elegans. Although the exact details of animal development can vary in bewildering ways depending on the species, all animal development exemplifies a common imperative: start with one cell, end with many different cells. In most animal species, development begins with the fertilized egg. Once the egg divides into its daughter cells, becoming an embryo, the organism begins heading toward a well-defined
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