The development of an animal from a fertilized egg requires the coordination of processes throughout the body. Imagine the problems that would arise if growth of the body were not symmetrical, or if the growth of organ systems did not match the demands placed on them by the growing body. Because hormones circulate in the blood to all parts of the body, they are ideal signals for integrating and coordinating the many diverse processes involved in development.
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Classic experiments revealed that hormones control insect metamorphosis.
Vertebrate sexual development is controlled by steroid hormones.
Human puberty results from a change in sensitivity of the hypothalamus to negative feedback.