WATER RELATIONS OF FROG 



119 



of intake, excretion of urine is prevented by more than the intake 

 missed; or paradoxically, water entrance appears to promote net 

 water elimination. 



Intermediate conditions between immersion in water and free- 

 dom from surrounding water were tested. Very shallow water or 

 wet towels allow nearly as much water intake as deep water, but 

 wet filter paper allows little or no water intake. Gradation of in- 

 take by this means is difficult to secure ; the phenomenon is almost 

 "all-or-none." 



It might seem justified to state that the "wetness of the skin" 

 makes the difference between the recovery of the frog in water 

 and the frog out of water. But this would credit to some one fac- 

 tor a situation that has many possible factors. The physical dif- 



+10 ^20 ^30 



To-tal Water Load 



Fig. 71. Eate of water exchange (% of Bo/hour) in relation to initial water load 

 (% of Bo), in frogs kept out of water in moist air. Intake is zero; output as urine 

 is indicated by points each of which is the mean of 10 tests. Kedrawn from Adolph 

 ('39b). 



ferences of hydrostatic pressure, temperature, texture of surface, 

 and many others, are all present. The physiological differences 

 may be myriad ; the differences are more than skin-deep, since urine 

 is not formed in the skin. So the two types of load are described 

 with fewest implications as belonging (b) to a frog sitting on a 

 dry wire screen in air saturated with moisture instead of (a) to a 

 frog sitting in a glass beaker with water 2 cm. deep. 



That a frog depends for constancy of water content upon being 

 in a wet environment is almost obvious. How much time does a 

 frog ordinarily spend out of water? Does it locate water more 

 quickly when already desiccated? Is the movement toward water 

 random or oriented? 



I have tested many frogs in water deficit to see how in the labo- 

 ratory they find their way to water. In the conditions tested, they 



