1. Secretory activity of the ventral head glands in embryos of Locustana pardalina and Locusta migratoria is necessary for inducing the moult and controlling subsequent events in late development.

2. Continued secretory activity of the glands for a period after the moult is required for further progress in development.

3. The development hormone released by the glands activates in turn the pleuropodial glands. The latter, in order to become fully functional, also require the continued secretory activity of the endocrine glands for a period after the moult.

4. The enzyme-substrate system responsible for the formation of melanin does not become active until the development hormone is liberated. However, the system was present in the egg beforehand in an inactive state and it is suggested that it is held in check by an inhibiting influence.

5. The amount of melanin deposited appears to be correlated with the amount of development hormone released.

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