Early-developing cells such as the organizer cells secrete local factors that make late-developing cells fix their fates and forms, and these cells, in turn, secrete factors that create organs and the connections between organs.IV The growth of an embryo is a process, a cascade. At each stage, preexisting cells release proteins and chemicals that tell the newly emerging and newly migrating cells where to go and what to become. They command the formation of other layers and, later, the formation of tissues and organs. And the cells within these layers themselves turn genes on and off, in
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