248 THE RATE OF GROWTH [ch. 



distributed in the tissues that water may be*. Land animals have 

 evolved from water animals with Httle change in this respect, 

 though the constant proportion of water is variously achieved. 

 A. newt loses moisture by evaporation with the utmost freedom, and 

 regains it by no less rapid absorption through the skin; while a 

 lizard in his scaly coat is less hable to the one and less capable of 

 the other, and must drink to replace what water it may lose. 



We are on the verge of a difficult subject when we speak of the 

 role of water in the living tissues, in the growth of the organism, 

 and in the manifold activities of the cell ; and. we soon learn, among 

 other more or less unexpected things, that osmotic equihbrium is 

 neither universal nor yet common in the living organism. The yolk 

 maintains a higher osmotic pressure than the white of the egg — so 

 long as the egg is living; and the watery body of a jellyfish, though 

 aot far off osmotic equilibrium, has a somewhat less salinity than 

 the sea-water. In other words, its surface acts to some extent as 

 a semipermeable membrane, and the fluid which causes turgescence 

 of the tissues is less dense than the sea- water outside!. 



In most marine invertebrates, however, the body-fluids con- 

 stituting the milieu interne are isotonic with the milieu externe, and 

 vary in these animals pari passu with the large variations to which 

 sea- water itself is subject. On the other hand, the dwellers in 

 fresh-water, whether invertebrates or fishes, have, naturally, a more 

 concentrated medium within than without. As to fishes, diiferent 

 kinds shew remarkable differences. Sharks and dogfish have an 

 osmotic pressure in their blood and their body fluids little diiferent 



* The vitreous humour is nearly all water, the enamel has next to 'none, the 

 grey matter has some 86 per cent., the bones, say 22 per cent. ; lung and kidney 

 take up mor^ than they can hold, and so become excretory or regulatory organs. 

 Eggs, whether of dogfish, salmon, frogs, ' snakes or birds, are composed, roughly 

 speaking, of half water and half solid matter. 



t Cf. {int. al.) G. Teissier, Sur la teneur en eau. . .de Chrysaora, Bull. Soc. Biol, 

 de France, 1926, p. 266. And especially A. V. Hill, R. A. Gortner and others. On 

 the state of water in colloidal and living systems. Trans. Faraday Soc. xxvi, 

 pp. 678-704, 1930. For recent literature see (e.g.) Homer Smith, in Q. Rev. Biol. 

 vn, p. 1, 1932; E. K. Marshall, Physiol. Rev. xiv, p. 133, 1934; Lovatt Evans, 

 Recent Advances in Physiology, 4th ed., 1930; M. Duval, Recherches. . .sur le milieu 

 interieur des animaux aquatiques, Thhe, Paris, 1925; Paul Portier, Physiologic des 

 animaux marins. Chap, iii, Paris, 1938; G. P. Wells and I. C. Ledingham, Effects 

 of a hypotonic environment, Journ. Exp. Biol, xvii, pp. 337-352, 1940. 



