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1 The Biological Laboratories, Harvard University, Cambridge 38, Massachusetts
1. By means of a standardized "pupal assay" for juvenile hormone, the endocrine activity of the corpora allata was found to undergo large and systematic changes during the postembryonic development of the Cecropia silkworm.
2. In each of the larval instars that were studied, the glands are least active just prior to the larval molt and most active shortly after the molt. Larval molting therefore appears to take place in the presence of a declining titer of juvenile hormone—an endocrine situation which apparently permits the striking changes in morphology and pigmentation which occur in successive larval stages of the Cecropia silkworm.
3. The corpora allata show low but definite activity at the time of pupation. If the glands are extirpated so that pupation occurs in the absence of juvenile hormone many larval tissues overleap the pupal stage and undergo precocious adult differentiation. Therefore, the low concentration of juvenile hormone in the mature larva plays a definite role in the endocrine control of pupation.
4. The corpora allata are inactive throughout the entire pupal stage. This inactivity persists during the first two-thirds of adult development. During the final week of adult development, the glands recover their endocrine function and are maximally active by the time of emergence of the adult moth.
5. The absence of juvenile hormone proves to be an obligatory feature of the initial phase of adult differentiation. If active corpora allata are implanted into a pupa so that adult development is initiated in the presence of juvenile hormone, the pupa develops and molts into a creature which is a mixture of pupa and adult. The higher the titer of juvenile hormone, the more extensive is the preservation of pupal characters.
6. Juvenile hormone is effective in blocking adult differentiation only when it is present during the first five days of adult development. Consequently, the target of juvenile hormone appears to be certain early events which are the normal reactions to ecdyson at the outset of adult development. If unopposed by juvenile hormone, these events commit the cells to developmental reactions accompanied by metamorphosis.
7. A comprehensive theory is presented for the action of juvenile hormone in the Cecropia silkworm. According to this theory, juvenile hormone modifies the cellular reactions to ecdyson by opposing the de-repression, de-coding or acting-out of fresh genetic information prerequisite for progressive differentiation but not prerequisite for growth in an unchanging state.
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