The existence of two series of mechanical resonance frequencies in human cancellous bone is apparently supported by other results. This effect may be a microstrain phenomenon since all

ABSTRACT.-A detailed analysis of the occurrence of parietal eye component of the pineal complex in families of lizards has revealed that those in which the parietal eye is absent in most genera (e. g. Gekkonidae, Teiidae) tend to be restricted to low latitudes, whereas families in which the parietal eye is present in most of the genera (e. g. Agamidae, Iguanidae) extend to higher latitudes. Those few genera which lack parietal eyes in the families where the very great majority have parietal eyes are confined to ranges rather close to the equator. The parietal eye may be important in thermoregulatory or reproductive adaptations at higher latitudes where seasons are more severely varied than at lower latitudes. A pineal body is reported to be absent in crocodilians, edentates and, perhaps, dugongs. All these animals tend to inhabit tropical regions. In contrast, seals and walruses have remarkably large pineal glands and are found at very high latitudes. These few fragments of information cannot be used to support any conclusions, but they do suggest that it might be productive to consider the pineal complexes of animals in the context of their geographical distribution.

Melatonin fulfills many of the criteria for classification as a hormone. Until recently it was considered to be elaborated exclusively by pineal organs. Melatonin synthesis by other tissues has been indicated but not demonstrated unequivocally. Trout retinas in a whole-organ culture system in vitro synthesized tritiated melatonin from a substrate containing tritiated serotonin. This raises the possibility that the trout retina is an endocrine organ.

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