116 PHYSIOLOGICAL KEGULATIONS 



net body weights reattained. Thirty-three frogs that had been 

 desiccated to diverse extents have CA ± 2.87% of Bo after 6 hours 

 of recovery, and ± 2.11% of Bo after 24 hours of recovery. Almost 

 the same difference (CA ± 2.22) is found among frogs kept in water 

 balance for 24 hours, and again among frogs recovering from ex- 

 cesses of water ; indicating that the over-all fluctuations of weight 

 are not accentuated by intervening desiccation or hydration. More- 

 over, the differences are in each of the three groups (deficits, ex- 

 cesses, controls) positive and negative in equal numbers. 



The exchanges labelled "intake through the skin" may conceiv- 

 ably be net intakes and not total intakes, since it is not known 

 whether water goes out through the skin concurrently as more 

 comes in through it. This possibility makes no difference in the 

 present study, any more than output through the kidneys depends 

 on whether there is ''reabsorption" as well as "filtration," and 

 whether there are any other processes intermediary to excretion. 



No significant partition of water exchanges among several paths 

 appears in frogs. Loss by evaporation is exceedingly small, for 

 the frog is nearly completely immersed and breathes air saturated 

 with moisture. Gain by metabolic formation of water amounts to 

 less than 0.01% of Bo/hour at 20° C. These quantities cannot be 

 represented at all on the coarse ordinates of figure 66. 



In brief, loads of water imposed on frogs by intraperitoneal in- 

 jection or by evaporative desiccation, lead to net exchanges appro- 

 priate to restoration of control contents. Many recoveries are 

 complete in 6 hours, all are complete in 24 hours. At numerically 

 equal loads, recovery by net intake is faster. In excesses, intake 

 continues at about the usual rate characteristic of balance while 

 output is appropriately modified; in deficits, output helps intake 

 to compensate, output significantly diminishing while intake is in- 

 creasing. Thus the frog uses three of the four possible modifica- 

 tions by which its exchanges may lead to rapid recoveries of water 

 content. 



§ 38. Variations 



Above I mentioned the variations that occur in the control state. 

 Single individuals are weighed at hourly (or other) intervals to 

 measure how much the contents and exchanges of water vary, and 

 body weights are found to fluctuate by CA ± 0.77% of its mean/hour 

 {103 tests, 19 individuals). Errors of measurement are insignifi- 



