COERELATIVES OP WATER CONTENT AND EXCHANGES 261 



(e) Different with physiological condition of organism. Ar- 

 bacia egg 'injured" by heat; at diverse ages. 



With the numerous criteria and conditions under which perme- 

 ability has been studied in mind, values of h in diverse species may 

 be compared, perhaps by means of the partial list tabulated by 

 Lucke et al. ('39). The highest are for erythrocytes (of man, 

 3.0 n/atmosphere minute) and for Pliascolosoma (2.1). Actually 

 many permeability coefficients fall in a restricted band of values 

 lying between 3 and 0.1. In freshwater organisms (Ameba), values 

 are as low as 0.02. Finally there are numerous aquatic organisms 

 such as certain insects, and fish eggs, that are nearly impervious 

 to water, and for which indeed exact permeability coefficients are 

 not known. 



In precise comparisons the reciprocal of permeability, the re- 

 luctivity, is a useful coefficient. It may be thought of as repre- 

 senting the resistance to water exchange per unit of exchanging 

 surface. 



Permeability coefficients have scarcely been computed for ani- 

 mals that are continually taking in water through body surfaces in 

 the stationary state of turnover. Such are frog (§37) and earth- 

 worm (§49). The frog takes in 0.53 milliliters per hour (table 

 21) through a surface of 120 square centimeters. The "osmotic" 

 pressure of the blood is 0.24 osmolar (Adolph, '27c, p. 329), whence 

 the permeability is 0.14 p/atmosphere minute. Having ascertained 

 that "osmotic" pressure inside the water-loaded body is approxi- 

 mately proportional to fraction of dry substance D/Bq, hence to 

 l/(Bo + AW), and observing figure 66, 1 conclude that the permea- 

 bility is constant in positive water loads but increases several fold 

 in negative loads, as though in deficits osmosis were less opposed. 

 The earthworm takes in 0.10 milliliters per hour (table 21) through 

 a surface of 24 square centimeters, its body fluid being 0.16 osmolar 

 (Adolph, '27, p. 56). The permeability is 0.20 n/atmosphere min- 

 ute, presumably increasing 5 fold or more in extreme water deficits. 

 The protozoa of § 52 and § 54 might be similarly treated if the 

 internal osmotic pressures of their substance were known by more 

 than guesses. If, conversely, a supposedly universal value for the 

 permeability to water be ascertained, perhaps from the rate of 

 shrinkage upon entering a hypertonic medium, then from the rate 

 of turnover the internal "osmotic" pressure may be crudely pre- 

 dicted (Kitching, '36). 



