Temperature and prolactin were studied as control factors i n the process of newt forelimb regeneration. Temperature was varied i n five steps from 10" to 30°C under otherwise constant environmental conditions. Longitudinal new growth and limb differentiation were observed. Temperature was found to have a profound effect upon both developmental processes. Maximal longitudinal new growth resulting from changes i n temperature alone occurred at 2 5°C with maximal limb differentiation a t 30°C. At each temperature studied, prolactin enhanced both new growth and limb differcntiation. With prolactin, maximum new growth also occurred at 25"C, but maximum limb differentiation occurred at 20°C. For our purposes optimum limb development was defined as the greatest new grow-th commensurate with full normal development. This optimum was reached by prolactin treated animals at 20°C when the regenerating limbs studied achieved 100% maximum normal differentiation and 87% maximum new growth. It was possible by employing a combination of temperature changes and hormone administration to control newt forelimb regeneration and produce a whole spectrum of limb development.
Forelimb regeneration studies using the newt, Notophthalmus uiridescens, were done under constant environmental conditions at 20' C. A series of three-month regenerative studies were done during a two and onehalf year period. Under conditions which eliminated factors known to affect regeneration, such as temperature and variation in light-dark cycle, a significant seasonal variation in the regenerative process was found. The rate and amount of regenerative new growth during the summer was enhanced over that in winter. A series of repetitive regenerations in the same group of animals demonstrated that this variation was cyclic under controlled environmental conditions for long periods of time. It is suggested that a cyclic biologic rhythm, probably endocrine in nature, is responsible for the phenomenon.
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