246 THE RATE OF GROWTH [ch. 



notably, while all the while the embryo continues to grow in bulk 

 and weight. But later on, and especially in the higher animals, the 

 water-content diminishes as growth proceeds and age advances; 

 and loss of water is followed, or accompanied, by retardation and 

 cessation of growth. A crab loses water as each phase of growth 

 draws to an end and the corresponding moult approaches; but it 

 absorbs water in large quantities as soon as the new period of 

 growth begins*. Moreover, that water is lost as growth goes on has 

 been shewn by Davenport for the frog, by Potts for the chick, and 

 particularly by Fehhng in the case of man. Fehhng's results may 

 be condensed as follows : 



Age in weeks (man) 6 17 22 24 26 30 35 39 



Percentage of water 97-5 91-8 92-0 89-9 86-4 83-7 82-9 74-2 



I 



The following illustrate Davenport's results for the frog: 



The following table epitomises the drying-off of ripening maize f; 

 it shews how ripening and withering are closely akin, and are but 

 two phases of senescence (Fig. 76): 



The bird's egg provides all the food and all the water which the 

 growing embryo needs, and to carry a provision of water is the 

 special purpose of the white of the egg ; the water contained in the 

 albumen at the beginning of incubation is just about what the 

 chick contains at the end. The yolk is not surrounded by water, 

 which would diffuse too quickly into it, nor by a crystalloid solution, 

 whose osmotic value would soon increase; but by a watery albu- 

 minous colloid, whose osmotic pressure changes slowly as its charge 

 of water is gradually withdrawn J. 



* Cf. A. Krogh, Osmotic regulation in aquatic animals, Cambridge, 1939. 

 t Henry and Morrison, 1917; quoted by Otto Glaser, on Growth, time and form, 

 Biolog. Reviews, xiii, pp. 2-58, 1938. 



X Cf. James Gray, in Journ. Exper. Biology, iv, pp. 214-225, 1926. 



