Water co 



untied, urine is voided and the weight falls to the original value. When the 

 concentration of the medium is increased, either by salt or sugar, the rate of 

 gain of weight is decreased and in hypertonic solutions weight is lost. Adolph 

 calculated that at 20°, water equivalent to 31 per cent of the body weight can 

 enter and be excreted every 24 hours. This is only a small fraction of what 

 would enter a true osmometer of similar concentration and surface. A skinless 

 frog gains weight faster than a normal frog. If the brain is destroyed or lesions 

 are made in the midbrain or anterior medulla, but not in the spinal cord, the 

 rate of water intake increases at once by as much as 4 to 5 times. Slime secre- 

 tion may be important in decreasing skin permeability to water. Adolph found 

 that a normal frog with cloaca open will gain weight for 25-50 hours when 

 placed in a very dilute solution of NaCl, but not in sucrose. Dilute NaCl 

 seems to stimulate water uptake above water excretion, even though in dis- 

 tilled water the same frog may maintain a constant weight. 



No amphibian has become totally independent of a moist medium, although 

 many do not lay their eggs directly in pond water, and some toads live as adults 

 in relatively dry air. Needham^-"' ^^^ lists the moisture-conserving adapta- 

 tions used by various Amphibia which do not lay their soft-coated eggs in pond 

 water; they deposit eggs in leaf mold, in foam in holes in the ground, or be- 

 tween leaves which form a cup to collect moisture. The rate of water uptake 

 by tadpoles during the first day after hatching is three times the rate in adults. 

 Some desert Amphibia develop very rapidly and the adults burrow under- 

 ground. '''^ One burrowing form, ChirolepUis platycephahis, "can store so 

 much water in its urinary bladder, subcutaneous tissues and peritoneal cavity 

 . . . that it is occasionally used by the Australian aborigine as a source of drink- 

 ing water." A few frogs are known to breed in brackish water, ^'*^ but none are 

 truly marine. 



The water content is similar in terrestrial and aquatic anurans, but the 

 terrestrial species survive greater loss of body water, as shown in Table 3. -^^ 



There have been numerous indications that the inward permeability of 

 the amphibian skin for water is greater than its outvvard permeability, but 

 experiments with heavy water on frogs ^^^ and on isolated frog skin -'" 

 led Krogh to conclude "that an irreciprocal permeability for water does not 

 exist in normal frogs," although some water may be carried along with salt 

 which is actively transported. 



In fresh water or in hypotonic solutions frogs, like most fresh-water fish, 

 do not drink water, but they absorb water through the skin; in hypertonic 

 solutions (140 mM NaCl), they do drink and produce a concentrated (but 

 blood-hypotonic) urine. Adolph^ showed that the skin is limited protec- 

 tion against loss of water by evaporation in air. At 100 per cent humidity 

 at 20°C. one fifth of the heat produced in the metabolism of the frog is lost 

 as latent heat of evaporation, hence the frog is continually at a higher 

 temperature than the air and could never take up water from saturated air. 

 However, water loss by way of the skin is reduced by high humidity. Lesions 

 to the midbrain and anterior medulla increase skin permeability to water, 

 possibly via the hypophysis. ^ 



The posterior lobe of the pituitary (neurohypophysis) acts to increase 

 the water content of amphibians. Injections of the active pituitary principle 

 caused increase in weight in amphibia according to species in the following 



