Chapter VI 



WATER RELATIONS OF FROG 



§ 36. If the study of water in organisms is to be general, the 

 same variables that are measured in mammals require to be corre- 

 lated in numerous other kinds of living beings. A frog has many 

 functional differences from a dog, and I now inquire whether and 

 in how far the pattern of water exchanges is different. Is the frog, 

 an animal usually immersed in water, less concerned with constancy 

 of water content? The species Rana pipiens is the one studied 

 except where otherwise stated. 



A frog does not drink water by mouth, but takes water into the 

 body through the skin. In older investigations {e.g., of Durig, '01) 

 this fact was ascertained by blocking the gullet in various ways. 

 Equally decisive is the fact that frogs in water gain weight con- 

 tinuously and at nearly steady rates in successive short periods of 

 time. When watched, no frog is seen to open the mouth or to swal- 

 low water during the gain of weight. The absence of muscular 

 movements in water intake seems to some persons to confer greater 

 automaticity upon the process in frog than in dog. 



A frog puts water out from body through the cloaca, all of it 

 presumably having passed through the kidneys. Only in types of 

 water load other than those studied here does water leave at signifi- 

 cant rates through the skin or any other path than the kidneys, so 

 far as is known. 



§ 37. Water exchanges 



Recoveries of water content are measured in frogs immersed in 

 water at 20° C. with nostrils in air. Excesses of water result from 

 sudden injection of distilled water into the peritoneal cavity. Defi- 

 cits are previously produced by 1 to 24 hours of evaporative desic- 

 cation, the frog being temporarily out of water in air (Adolph, 

 '39b) . Increments of body weight represent the loads of water. 



Figures 64 and 65 indicate the conduct of tests. When the frog 's 

 cloaca is closed by ligation the weight increases, corresponding to 

 the rate of entrance of water through the skin. Whenever the cloaca 

 is later opened, urine escapes and the amount of it that has formed 

 since ligation is thus ascertained. When the cloaca is not ligated, 

 changes of body weight indicate net exchanges of water. 



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