The first thing any embryo must do before it can develop into an organism is establish early polarity—that is, it must set up a way to distinguish its top from its bottom and its back from its front. The mechanisms that establish the earliest spatial configurations in an embryo are far less to be quite different from the polarity signals in the development of humans and other mammals.
In the fruit fly, polarity is established by signals inscribed in the yolklike cytoplasm of the egg before fertilization, so that when the sperm contributes its genetic material, everything is already set to go. Given all the positional information that must be distributed throughout the cell, it takes a fruit fly a among cells. Yet how polarity is established in mammals is currently a tempting mystery to researchers.
Once an embryo establishes polarity, it relies on sets of essential genes that are remarkably similar among all life forms for elaboration of its parts. There is an astonishing conservation of mechanism in this process: the genes that help make eyes in flies are similar to the genes that make eyes in or extremities and become identifiable as distinct species, the developmental mechanisms they use are remarkably similar.
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