tion tending to keep the cell isotonic is water exchange. We shall consider the 

 role of the contractile vacuole in marine Protozoa later. 



Several gregarines from the gut of mealworms swell and shrink according 

 to the tonicity of the medium." ' The gregarines have a high content of 

 glycogen, which may cause as much as 70 to 80 per cent of their volume to 

 be osmotically inactive. 



The organisms which have been most studied in respect to their ability to 

 swell or shrink in changing tonicity are the eggs of marine invertebrates, 

 particularly echinoderms and annelids. In general, when the volume of a 

 marine invertebrate egg is measured in different dilutions and concentrations 

 of sea water the cell is found to be a good osmometer, that is, it follows Boyle's 

 law over a narrow range: pressure (P) X volume (V)=ra constant (K), 

 (PV=:K). In very dilute sea water it does not swell as much as would be 

 expected if it followed the gas laws in proportionality of volume and pressure 

 (Fig. 3). The explanation ^^^ of this deviation seems to be that a certain 

 portion of the cell volume is osmotically inactive and thus does not take up 

 water. Large protein and fat molecules, together with bound water, are 

 osmotically insignificant, yet occupy a much greater relative volume than in- 

 organic salts. When a correction is made for this osmotically inactive volume 

 (b) and the corrected osmotically active volume (V - b) is plotted against 

 external pressure, the gas laws as applied to dilute solutions are obeyed (Fig. 

 3). The osmotically inactive volume --^ is 7.3 per cent of the initial cell 

 volume in unfertilized Arbacia eggs, 27.4 per cent in fertilized eggs. ^-^ 

 Isolated nuclei of Arbacia eggs conform better to the gas laws;^^ the osmo- 

 tically inactive volume is negligibly small. 



A second explanation of the failure of cells to swell as much as predicted in 

 dilute media might be leakage of salt., i.e., failure of the semipermeable nature 

 of the membrane. Salt leakage apparently does not occur or is negligible over 

 the range of rapid reversible changes, but it may be important in extreme 

 dilutions, where injury occurs. PermeabiHty to salt is very low in most devel- 

 oping aquatic embryos. ^^^ 



In Protozoa which lack contractile vacuoles and in marine eggs, then, the 

 rate of gain or loss of water is sufficiently great to provide a simple method of 

 adjustment to changing external concentration. 



Multicellular Animals. Osmotic adjustment by volume change is also 

 shown by the marine sipunculid worm, Phascolosoma, whose body surface 

 behaves like a semipermeable membrane. This worm is able to survive indefi- 

 nitely in concentrated (160 per cent) sea water. « Approximately 23 per cent 

 of the body volume of Phascolosoma is osmotically inactive. The body weight 

 decreases or increases rapidly on transfer to a medium of different tonicity and 

 reaches equilibrium in 2 to 8 hours. On return to normal sea water after 

 hydration or dehydration, the original volume is reached in a few hours. 

 The rate at which water enters exceeds the rate at which it leaves. Apparently 

 there is little or no salt transfer because the volume of the animal remams high 

 or low, according to the concentration of the medium, for several days. When 

 salt solutions are injected, similar volume changes occur correspondmg to the 

 concentrations injected; some days later there is a tendency for body volume to 

 return toward normal, indicating that there may be a very slow and delayed 



