Summary(i) An injection of neurohypophysial extracts into frogs kept in water produces a temporary increase of body weight. This rise of weight (the ‘water‐balance effect’ or ‘Brunn reaction’) has beēn shown to be due to an increase of the body water. (2) The water‐balance effect has been clearly demonstrated in various species of amphibians only. It does not occur in teleost fishes or in mammals. Its occurrence in reptiles is doubtful and requires reinvestigation. It has not been investigated in cyclostomes or birds. (3) Evidence concerning the mechanism of the amphibian water‐balance effect may be summarized as follows, (a) The evidence for an increased water uptake through the skin following an injection of neurohypophysial extract appears satisfactory. However, further analysis of this extrarenal effect seems desirable. (b) Evidence for a renal inhibitory effect is suggestive but not unequivocal, (c) Evidence for a direct effect on the tissue imbibition is lacking. (4) The specificity of the neurohypophysial water‐balance effect with particular reference to the effect of other endocrine glands has been discussed: certain steroid hormones have been reported to produce slight increases of body weight in amphibians, but the mechanism of these increases has not been investigated. (5) Data referring to the influence of other hormones on the water‐balance effect of neurohypophysial extracts have been considered: it is pointed out (a) that in most of the experiments concerned the endocrine preparations were not “homologous, and (b) that the majority of the experiments were of very short duration. However, it has been shown that treatment with thyroxin for several days enhances the neurohypophysial water‐balance effect in winter frogs. (6) Subtotal hypophysectomy has been observed to produce polyuria in frogs and toads kept in water; the body weight of hypophysectomized animals is not significantly affected. Hypophysectomized frogs kept in dry surroundings do not seem to lose water more readily than normal controls. However, the probably unavoidable hypothalamic lesion and the removal of the adenohypophysis in the course of the operation obscures the issue. (7) There are not sufficient data to permit the formation of a theory regarding the physiological significance of the neurohypophysial water‐balance effect in amphibians. However, the fact that frogs in dry surroundings are less liable to lose water when injected with the oxytocic fraction of mammalian posterior pituitary extracts may be considered as supporting the suggestion that the response to the neurohypophysial water‐balance principle is an adaptation to an amphibious life. The absence of a water‐balance effect in wholly aquatic animals (fishes) and wholly terrestrial animals (mammals) may be noted in this connexion. (8) The principle producing a temporary increase of body water in amphibians (water‐balance principle) has been found in the neurohypophysis of all vertebrate classes investigated. The content of water‐balance principle has been quantitatively determined in the pituitary glands of elasmobranchs, marine and freshwater teleosts, amphibians, reptiles, birds and mammals. Cyclostomes have not been examined. (9) The water‐balance principle has been shown to be not identical with the mammalian posterior pituitary antidiuretic hormone and to be mainly contained in the oxytocic fraction. Its identity wit
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